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Part L

Approved Document L: Conservation of Fuel and Power

Conservation of fuel and power (energy efficiency)

Read on gov.uk All Approved Documents

Also known as

Part LApproved Document LAD LConservation of Fuel and PowerApproved Document L Volume 1AD L Vol 1

What Part L requires for a home project

The Part L requirements and terms the buildwiz guides rely on, with paragraph references where the document gives one. Check the current edition on gov.uk before you build to it.

Limits and figures

Approved Document L: thermal element renovation threshold
Trigger point under Approved Document L for upgrading a renovated thermal element. If more than 50% of the surface of an individual thermal element (such as a roof, external wall, or ground floor) is renovated, the whole element should be improved to the U-value in Table 4.3 column (b). Separately, a major renovation (more than 25% of the surface area of the whole external building envelope) triggers the same upgrade. Widely overlooked on full re-roofs where the homeowner assumes only the new extension section is inspected; building control must be notified.
Ceiling-level loft insulation total depth (Part L)
Total depth of mineral wool loft insulation at ceiling joist level to hit the 0.16 W/m²K target for existing-dwelling loft upgrades under Approved Document L 2021. Installed as two layers: 100mm between the joists and 170mm laid perpendicular over the joists so the timber is covered and not acting as a thermal bridge. For new extensions the roof U-value target drops to 0.15, which typically needs warm-roof PIR rather than between-joist mineral wool alone.
Dwelling floor area above which Part L consequential improvements can apply
Regulation 28 and Approved Document L Section 12 (paras 12.1, 10.11) only require consequential improvements where an existing dwelling has a total useful floor area over 1,000m2, measured to the internal face of external walls across all enclosed spaces. A normal house extension is far below this.
Existing rooflight U-value above which replacement is required (change of energy status)
Approved Document L Vol 1 para 11.7: existing windows, roof windows and rooflights with a U-value above 3.80 W/(m²K) should be replaced to meet the limiting standards when a space such as a loft becomes part of the heated dwelling.
Existing window and door U-value above which replacement is required (change of energy status)
Approved Document L Vol 1 para 11.7: existing windows and doors separating conditioned from unconditioned space with a U-value above 3.30 W/(m²K) should be replaced to meet the limiting standards when a space such as a loft becomes part of the heated dwelling. Both conditions apply: the fitting separates conditioned from unconditioned space, and its U-value is above the threshold.
Extension glazing area limit (elemental method)
Under the elemental route in Approved Document L para 10.7(d), the total area of windows, roof windows, rooflights and doors in an extension should not exceed 25% of the extension floor area, plus the area of any openings that no longer exist or are no longer exposed because of the extension. Larger glazed runs are possible via the area-weighted U-value or SAP notional-extension routes (paras 10.9 and 10.10) with trade-offs.
Extension ground floor maximum U-value (Part L 2021)
Maximum U-value for new extension ground-floor constructions in England under Building Regulations Approved Document L 2021. The PIR thickness needed depends on the floor's perimeter-to-area ratio, so take it from the U-value calculation rather than assuming a standard board.
Extension pitched/flat roof maximum U-value (Part L 2021)
Maximum U-value for new extension roof constructions in England under Building Regulations Approved Document L 2021. Usually requires insulation both between and below the rafters (or a combination of cavity, loft-level and warm-roof layers on flat roofs), since a single layer rarely achieves the 0.15 target.
Extension wall maximum U-value (Part L 2021)
Maximum U-value required for new extension external walls under Building Regulations Approved Document L 2021. A significant increase in thermal performance over the previous 0.28 W/m²K requirement. In practice, requires a cavity of about 150mm or more: full-fill mineral wool batts, or partial-fill PIR boards with a residual cavity, often with an insulating blockwork inner leaf.
Loft ceiling-level U-value target (Part L existing dwelling upgrade)
U-value target for upgrading insulation at ceiling joist level in an existing dwelling loft under Approved Document L 2021 Table 4.3 (renovations and thermal element upgrades). Separate from the 0.15 W/m²K target applied to new extension roofs and the 0.11 W/m²K new-dwelling target. Typically achieved with 270mm mineral wool laid in two layers.
Maximum U-value for roof windows in extensions
Pitched roof windows in extensions: max 1.4 W/m²K (AD L 2021 Table 4.2, assessed as windows). Flat rooflights: max 2.2 W/m²K using developed surface area (Ud-value) methodology. High-performance triple-glazed Velux/Fakro units achieve 1.2 W/m²K or better.
New dwelling ground floor limiting U-value (Part L 2021)
Approved Document L Volume 1 (2021 edition incorporating 2023 amendments), Table 4.1, sets a limiting floor U-value of 0.18 W/m²K for new dwellings. The notional dwelling in Table 1.1 uses 0.13 W/m²K; this is not the universal limiting value. Whole-dwelling targets may require better performance, and insulation thickness depends on the complete build-up. Apply the edition and transitional provisions governing the project.
Notice of heating commissioning to building control
Approved Document L para 8.5 says the notice of completion of commissioning goes to the building control body within five days where a building notice or full plans have been given, and within 30 days in other cases such as work by a competent person scheme installer. Until it is received the building control body may decide not to issue the completion certificate (para 8.7).
Part L 2021 uplift effective date for new building control applications
Date from which Approved Document L Volume 1 (2021) applied to new building control applications and building notices in England. Projects with a valid full plans application or building notice deposited before this date could continue under the previous Part L 2013 standards (subject to a transitional period requiring work to start within 12 months). After this date, the tighter limits for new elements in existing dwellings (walls 0.18, roof 0.15, floor 0.18, Approved Document L Volume 1 Table 4.2) became mandatory.
Part L renovation threshold: exceed this share of an individual thermal element
Approved Document L Volume 1 (2021/2023), paragraph 11.3, applies to renovation of more than 50% of an individual thermal element, or a major renovation affecting more than 25% of the building envelope. Paragraph 11.4 explains the area to assess. Table 4.3 gives element-specific improvement values, subject to feasibility provisions; the 0.30 W/m²K value is not a universal target for walls, floors and roofs.
PIR flat-roof minimum thickness for Part L 2021 compliance
PIR thickness typically needed (lambda 0.022 W/mK, over an 18mm ply or OSB deck) to meet the 0.15 W/m²K limiting U-value for a new flat roof on an extension in England (Approved Document L 2021 Volume 1 Table 4.2) in a warm-deck build-up. 120mm only reaches about 0.18 W/m²K. Some building control guides specify 150mm. The exact figure depends on the deck, fixings and product, so take it from the manufacturer's U-value calculation.
PIR insulation compressive strength: floor-grade boards
Compressive strength of floor-grade PIR (notably Kingspan ThermaFloor TF70), specified for use under screed where the inspector wants application-certified product. Not required by Approved Document L for domestic floors: 120 kPa general-purpose boards also pass, but floor-grade avoids rejection on sites where the BCO insists on an application-certified board.
PIR insulation R-value at 100mm thickness
Declared thermal resistance of a 100mm PIR board with lambda 0.022 W/mK. The standard thickness for solid-floor insulation under screed in English extensions: achieves approximately 0.17 W/m²K against the Part L 2021 extension floor limit of 0.18 W/m²K.
Pitched-roof PIR insulation below rafters: typical thickness
Typical PIR board or insulated-plasterboard thickness fitted below the rafters to top up the between-rafter insulation layer. Eliminates thermal bridging through the rafter timber and lets a 100-120mm between-rafter layer hit the Part L roof U-value target.
Pitched-roof PIR insulation between rafters: typical thickness
Typical PIR board thickness installed tight between rafters in a pitched warm roof extension. Combined with a below-rafter PIR layer this build-up hits the Part L 2021 0.15 W/m²K roof U-value target without needing deeper rafters. Actual thickness depends on rafter depth and the below-rafter top-up.
Pre-2022 Part L floor U-value target for new thermal elements in existing dwellings
Target U-value for the ground floor of a new extension under the Part L 2013 regime that applied to designs submitted before 15 June 2022. The current target tightened to 0.18 W/m²K, requiring thicker under-slab PIR (typically 100–150mm rather than 75–100mm). Where a pre-2022 specification has already been quoted by the screeder or groundworker, the BCO will require the higher thickness regardless of the original quote.
Pre-2022 Part L roof U-value target for new thermal elements in existing dwellings
Target U-value for the roof of a new extension under the Part L 2013 regime that applied to designs submitted before 15 June 2022. The current limiting value for a new extension roof is 0.15 W/m²K (Approved Document L Volume 1 2021, Table 4.2), driving the typical PIR thickness up from around 100mm to 150mm. Designs drawn before the uplift but built afterward are commonly forced into a mid-build upgrade by the BCO at the structure inspection.
Pre-2022 Part L wall U-value target for new thermal elements in existing dwellings
Target U-value for new external walls of an extension under the Part L 2013 regime that applied to designs submitted before 15 June 2022. Significantly looser than the current 0.18 W/m²K, so a typical pre-2022 wall achieved compliance with around 100mm of cavity PIR. The June 2022 uplift drove a step-change in cavity widths and PIR thicknesses; designs caught mid-flight are the most common source of mid-build insulation upgrades flagged at the structure or pre-plaster inspection.
Solid wall renovation U-value target (Part L 2021)
Approved Document L Volume 1 (2021 edition incorporating 2023 amendments), Table 4.3, gives 0.30 W/m²K for an existing wall upgraded with internal or external insulation, not for every thermal element. Paragraph 11.3 applies where more than 50% of an individual element is renovated, or a major renovation affects more than 25% of the building envelope. Technical, functional and economic feasibility provisions apply; assess moisture risk when selecting the insulation system.
Triple glazing typical whole-unit U-value
Representative whole-unit U-value for a triple-glazed bifold, sliding door or window from a mainstream UK supplier. Better than the Approved Document L 2021 window ceiling of 1.4 W/m²K and the 1.2 W/m²K design target, so using triple glazing usually provides SAP assessment headroom. Not a regulatory figure: a typical product-level value used to justify spec upgrades where energy performance matters.
UFH ground floor PIR insulation: building regulations minimum
Typical minimum PIR insulation thickness above the slab when underfloor heating is laid on a ground floor. It is manufacturer and installer guidance, not a figure Approved Document L sets: the regulation limits heat loss from an underfloor-heated ground floor to 10W/m² (para 6.29) and sets the floor U-value for extensions (Table 4.2). The thickness that achieves both depends on the floor, so the designer or insulation supplier confirms it.
UFH ground floor PIR insulation: recommended thickness
Common upgraded PIR thickness above the slab on UFH ground floors, giving better thermal performance and an easier compliance margin against the 0.18 W/m²K Part L target than the 70mm usual working minimum (Part L sets a floor U-value, not a thickness). The extra 30mm pulls the slab pour deeper but reduces lifetime heating costs and gives quicker response from the UFH circuit.
UFH screed thickness trigger for mandatory night/unoccupied setback control
Screed thickness above which Approved Document L asks a new underfloor heating system to reduce the room temperature automatically at night or when the room is unoccupied. Thicker screeds store more heat and respond slowly. The other controls for new underfloor heating in the same paragraph (adjustable operating temperature, manual override on electric systems) apply at any thickness.
Wet underfloor heating maximum flow temperature
Maximum design flow temperature for a newly installed or fully replaced wet heating system, underfloor heating included (Approved Document L 2021 para 5.10). A thermostatic blending valve on the manifold mixes boiler flow down to a screed-safe 35-45°C.
Window whole-unit U-value design target
Common specification target for the whole-window U-value (frame + glass); not a regulatory figure. The Approved Document L limit for a new window in an existing dwelling is 1.4 W/m²K or Window Energy Rating Band B (Table 4.2); 1.2 W/m²K is the notional-dwelling window value in Table 1.1, used in new-build calculations. Most aluminium and uPVC double-glazed systems from reputable manufacturers meet this comfortably.

Terms explained

Air permeability
The rate at which air leaks through a building's envelope at a 50 Pa pressure differential, measured in m³/(h·m²) of envelope area. Approved Document L 2021 sets 8 m³/(h·m²) as the maximum for new dwellings and a 5 m³/(h·m²) notional target. Tested by a registered air-tightness tester using a blower-door fan.
Air permeability test
A pressurisation test measuring air leakage through a building envelope, expressed as m³/(h.m²) at 50 Pa. Approved Document L Volume 1 (2021 edition incorporating 2023 amendments), Section 7, requires testing of new dwellings. It does not set a general extension size or glazing threshold for testing. For extensions, agree evidence for the compliance route in paragraphs 10.7–10.10; assess any test result against the applicable project target rather than a universal default.
Aluminium Foil Tape
Self-adhesive aluminium tape used to seal joints between PIR insulation boards, completing the vapour control layer. Untaped joints let air through and reduce effective thermal performance.
Area-weighted U-value method
An alternative route in Approved Document L para 10.9 for an extension with more glazing than the 25% allowance: show that the area-weighted average U-value of all the extension's thermal elements, sum of (U x A) divided by sum of A, is no worse than an extension of the same size and shape that complies with the elemental route. Agree the route with building control before ordering glazing.
Benchmark Commissioning Checklist
The industry-standard boiler commissioning logbook published by HHIC. Approved Document L (paragraph 8.8) accepts a completed checklist as evidence that the system was commissioned properly. It documents system flush, inhibitor addition, gas pressure readings, safety device tests, and radiator balancing. Building control may request it at final inspection.
Bespoke large-format aluminium rooflight
Made-to-order large-format fixed aluminium rooflights supplied by specialist glazing companies. Substantially different from standard Velux pivot windows: installed by the specialist supplier rather than the roofer, cost £10,000-15,000 for large units, 8-16 week lead time. May require 30-minute fire resistance glazing if over escape routes. A documented 55m² kitchen extension in Oxfordshire used two large units from a specialist supplier.
Blinding Concrete
Thin layer (50-75mm) of weak concrete (C8/10) poured over excavated ground to provide a clean, level surface for placing reinforcement and DPM. Halves the required reinforcement cover from 75mm to 40mm under Eurocode 2.
Boiler interlock
A control arrangement that stops the boiler firing when no room thermostat, cylinder thermostat or other control is calling for heat, so it does not run (or short-cycle) when nothing needs heating. Approved Document L para 5.25 says that if work is carried out on a system that includes a boiler, a boiler interlock should be installed, which is the clearest existing-system control rule when emitters are added for an extension.
Cavity closer
Prefabricated insulated frames installed at window and door openings to close the cavity and prevent thermal bridging. They help meet Parts C (moisture) and L (thermal) of the Building Regulations; the 0.45 m²K/W minimum thermal resistance comes from the Accredited Construction Details, not Part L itself, and in a masonry house wall Approved Document B (Diagram 5.3) asks only that the cavity is closed around openings, with no fire rating for the closer. First-fix installation during wall construction is required by NHBC/LABC standards.
Cavity Wall Insulation Batts
Semi-rigid mineral wool or glass fibre batts designed to partially fill the cavity in a masonry cavity wall. Held in place against the inner leaf with retaining clips. Must leave a minimum 50mm clear cavity to prevent moisture bridging.
Change to energy status
When a previously unheated or exempt space becomes part of the heated dwelling, such as a loft or garage conversion, the energy efficiency requirements apply to that space. Approved Document L Vol 1 para 11.6 gives the loft conversion example, and para 11.7 requires retained thermal elements and poor existing windows to be upgraded.
Compressible expanding tape
Specialist perimeter sealing tape for window and door frames that expands to fill gaps, providing superior thermal, acoustic, and weathertight performance compared to foam alone.
Concrete Blocks 100mm (Dense)
Standard 100mm dense concrete blocks used for load-bearing walls and below-ground work. Heavier than aerated alternatives but stronger in compression.
Concrete Blocks 100mm (Lightweight/Aerated)
Lightweight aerated concrete blocks (e.g. Thermalite, Celcon) used for inner leaf of cavity walls. Better thermal performance than dense blocks and easier to cut and handle.
Concrete Blocks 140mm
Wider concrete blocks used where additional structural strength or wall thickness is required, common in foundation walls and below DPC level.
Concrete Roof Tiles
Machine-made concrete tiles available in a wide range of profiles and colours. Heavier than clay tiles but cheaper and very durable. Common for matching existing roofs on 1960s-1990s properties.
Condensing boiler
The gas appliance installed at second fix plumbing: the primary cost driver and the item that triggers the Gas Safe notification requirement. All new gas boilers installed in the UK must be condensing (high-efficiency) by regulation.
Consequential improvements
Extra energy-efficiency upgrades that Building Regulations regulation 28 and Approved Document L Section 12 can require alongside certain work on an existing building. For dwellings the trigger applies only where the total useful floor area is over 1,000m2, so it does not apply to a typical house extension. Not the same as consequential losses in a contract dispute.
Controlled fitting
In Approved Document L, a window, roof window, rooflight or external door supplied as an entire unit including its frame. Only a complete new unit has to meet the Table 4.2 limits; replacing the glazing alone, or a sash in its existing frame, does not count (para 10.6).
EPDM Rubber Membrane
Single-ply rubber membrane for flat roofs. Excellent lifespan (30-50 years), flexible and easy to install as a single sheet without joints. The preferred flat roof covering for extensions. Requires a clean, smooth substrate (typically OSB or plywood deck).
EPS Insulation (Expanded Polystyrene)
Lightweight, rigid expanded polystyrene insulation boards. Lower thermal performance per mm than PIR but significantly cheaper. Commonly used below floor slabs and in less thermally demanding applications.
Flow temperature
The temperature at which hot water leaves the boiler or heat pump. Approved Document L 2021 (para 5.10) asks for a newly installed or fully replaced wet heating system to be sized to run at a maximum flow temperature of 55°C or lower, or the lowest temperature that still meets the home's heating needs where that is not feasible. Underfloor heating runs well at 35-55°C.
Frame sealant
Acrylic or low-modulus sealant used to seal the vertical DPC at the junction between new and existing walls, and to weatherproof DPC at external corners. Distinct from silicone sealant used in kitchens and bathrooms.
Grey Graphite EPS
Graphite-enhanced expanded polystyrene achieving lambda 0.030 W/mK vs 0.038 W/mK for standard white EPS. Provides 20-25% thickness saving at a small cost premium.
GRP edge trim
A preformed GRP or plastic trim bonded along roof edges and gutters to form a neat, watertight drip edge for a fibreglass roof.
GRP Fibreglass Flat Roof System
Glass-reinforced polyester flat roof system applied as a liquid resin and glass fibre matting that cures to form a seamless, rigid, waterproof surface. Very durable (25+ years) but more skill-intensive to apply than EPDM. Best installed by a specialist.
Heating zone
A conditioned area of a building on a single floor with the same thermal characteristics and temperature control requirements throughout (Approved Document L Appendix A). Approved Document L para 5.21(b) lets two adjacent rooms of similar function and heating needs, such as a kitchen and utility room, be treated as one zone, the case a rear extension most often meets.
Insulated Plasterboard (Thermal Laminate)
Plasterboard factory-bonded to a layer of PIR insulation. Provides insulation and a plaster-ready surface in one product. Used for internal wall insulation, particularly on solid masonry walls where cavity insulation is not possible.
Insulation
Thermal insulation products used in walls, roofs, floors and around openings to meet Building Regulations Part L energy efficiency requirements.
LED downlight
The dominant ceiling lighting fitting for kitchen extensions, recessed into plasterboard. Fire-rated versions required where the ceiling is beneath a bedroom or bathroom. Typical extensions specify 20-29 downlights.
Lintel
A horizontal structural element placed above openings (doors, windows) to transfer the load from above to the sides. Can be concrete, steel, or stone. Building control inspects that lintels are correctly sized and installed over every opening.
Liquid anhydrite screed
Pumpable self-levelling calcium sulphate screed. Nearly twice the thermal conductivity of sand-cement. Preferred for UFH. Requires laitance removal. Not suitable for wet rooms.
Loft Leg
Plastic or metal stilts enabling loft storage boards to sit above mineral wool insulation without compressing it. Standard solution to the compression problem in lofts.
Marmox thermal block
Thermally broken masonry block (lambda 0.27 W/mK vs 1.13 for dense concrete block) used at wall junctions to interrupt thermal bridges where extension walls meet the existing house structure.
Mesh Reinforcement A142
Light-duty welded steel mesh used to reinforce ground floor concrete slabs. A142 is the lightest standard mesh and is sufficient for most domestic floor slabs.
Mesh Reinforcement A193
Medium-duty welded steel mesh for floor slabs requiring greater reinforcement, such as where ground conditions are poor or spans are longer.
Notional extension method
The Approved Document L para 10.10 route: a SAP assessor shows that the dwelling plus the proposed extension has no worse primary energy, emission and fabric energy efficiency rates than the dwelling plus a notional extension of the same size and shape that complies with the elemental route. Allows glazing trade-offs beyond the 25% allowance, at the cost of a SAP calculation.
Perimeter Edge Strip
10-20mm compressible foam strip installed at room perimeter before screed pour over PIR floor insulation. Allows thermal expansion and prevents cold bridging at wall-floor junction.
Pipe Lagging
Foam tube insulation for pipes. The Water Fittings Regulations 1999 require pipes in unheated spaces to be protected against freezing, and Approved Document L (paragraph 4.24) asks for heating primary pipes outside heated spaces, all hot water primary and secondary circulation pipes, and the first metre of pipe from a cylinder to be insulated. Keeping cold pipes below 25°C is mainly about routing them away from heat sources.
PIR Insulation Board 100mm
100mm polyisocyanurate rigid insulation board. Commonly specified for floor and roof insulation in extensions to achieve current Building Regulations U-value targets.
PIR Insulation Board 120mm
120mm polyisocyanurate rigid insulation board. Used where maximum thermal performance is needed, particularly in flat roofs and floors where Part L 2021 U-value requirements are stringent.
PIR Insulation Board 50mm
50mm polyisocyanurate rigid insulation board with foil facing. High thermal performance for its thickness. Used in walls, roofs and floors. Common brands include Celotex and Kingspan. Standard board size 2400x1200mm.
PIR Insulation Board 75mm
75mm polyisocyanurate rigid insulation board. Often used in combination with other thicknesses to achieve the required U-value specified by building control.
Pitched roof window
Standard pivot-type roof windows (Velux GGL, Fakro FT series, RoofLITE equivalent) for pitched roofs. Require a specific flashing kit matched to tile type, structural trimmer rafters, and a manufacturer-specified underfelt collar. Available in standard size codes from builders merchants with 1-2 week lead times for stock sizes.
Pressure, temperature and flow units
The three units on boiler, water and gas datasheets. Bar measures the pressure of water or gas in a pipe (UK mains water is usually 1 to 4 bar, with a 1 bar statutory minimum at the boundary); low pressure is why an upstairs shower can feel weak. Degrees Celsius (degC) sets heating flow temperatures, the curing and frost windows for concrete and screed, and the 55degC Part L limit for a new boiler. Litres per minute (L/min) is flow rate, how fast water is actually delivered; it depends on pressure and decides whether a combi boiler can supply enough hot water. Heating output is quoted in kW, shared with the electrical units page.
Proprietary vapour control membrane
Purpose-made multi-layer vapour control membrane (e.g. DuPont Airguard Control, Pro Clima INTELLO) with targeted vapour resistance (sd 5-18m). Meets NHBC Standards 6.2.12 UKAS certification requirement, unlike cheap polythene sheeting.
Push-Fit Pipe 22mm (Speedfit)
22mm plastic push-fit pipe for larger supply and heating runs. Same advantages as 15mm push-fit: fast installation, no soldering, demountable joints.
Radiator
The primary heat emitter fitted at second fix plumbing. Each radiator is hung, connected to flow and return pipes, fitted with a TRV and lockshield valve, and bled. Designer/column radiators hold larger water volumes than standard convectors and can cause heating system capacity issues.
SAP assessment
Standard Assessment Procedure energy calculation, into which window U-values feed. An extension needs one only when it uses the Approved Document L 2021 para 10.10 route, typically when glazing exceeds 25% of its floor area plus the area of openings removed; the para 10.9 area-weighted U-value calculation is the other alternative.
Sheep's wool insulation
Natural fibre insulation alternative to mineral wool with lambda 0.035-0.040 W/mK. Better vapour permeability and moisture buffering than synthetic alternatives but carries a 2-3x cost premium.
Structural steel
Steel beams used to support loads in building construction, including lintels, ridge beams, and support frames for openings in walls.
System flush and inhibitor
Before a heating system is commissioned, all flux residues and swarf from new pipework must be flushed out and a corrosion inhibitor added: called for by Approved Document L (paragraph 8.8), which asks for cleaning, flushing and inhibitor to the BS 7593 code of practice, and a condition of most boiler manufacturers' warranties.
Thermal calculations
Calculations demonstrating that a building extension meets the thermal performance requirements of Building Regulations Part L, including U-values for walls, roof, floor, and windows.
Thermal element
A wall, floor or roof that separates heated inside space from the outside or from an unheated area. Building Regulations Part L sets minimum insulation (U-value) standards whenever a thermal element is newly built, replaced or substantially upgraded.
Thermostatic radiator valve
Self-regulating valve on a radiator that controls room temperature by adjusting hot water flow. Where Approved Document L 2021 asks for thermostatic room controls (new dwellings, or when the boiler is replaced: para 5.20), TRVs on every radiator outside the room with the thermostat are one way to meet it (para 5.22). They should not be fitted in the same room as the thermostat.
Thermostatic room controls
Controls that let the heating output be adapted separately in each room (or heating zone) served by a boiler or other heat generator, using a room thermostat plus thermostatic radiator valves elsewhere, individual room or zone thermostats, or networked emitter controls. Approved Document L para 5.20 requires them in new dwellings and when a heat generator is replaced; extending emitters alone is not a named trigger, so they are good practice rather than a rule for most extensions.
U-value
A measure of heat loss through a building element; lower is better
Ud-value
Developed surface area U-value used for flat rooflights (as opposed to pitched roof windows which use standard U-values). AD L 2021 Table 4.2 sets different limits: 1.4 W/m²K for pitched roof windows assessed as windows, 2.2 W/m²K for flat rooflights using Ud-value methodology.
UFH pipe
16mm PEX-a or EVOH barrier pipe supplied in coils of 100-200m, laid in continuous loops for wet underfloor heating systems and pressure-tested before screed.
Unbonded screed
Screed laid over DPM or insulation without bonding to the slab. Minimum 75mm thickness per BS 8204-1. Standard for modern extensions with Part L compliance.
Underfloor heating manifold
Central distribution point for wet underfloor heating loops, typically wall-mounted 400-600mm above finished floor level and connected to the heat source before screed is poured.
Warm roof
Flat roof construction where insulation sits above the structural deck, keeping the structure warm. Preferred modern approach. Requires 120mm+ PIR for Part L 2021 compliance.
Wet heating system
A heating system where a boiler or heat pump heats water that is pumped round the dwelling to emitters such as radiators or underfloor pipework (Approved Document L Appendix A). It is the term Approved Document L uses when it applies flow temperature and control rules, and it separates radiator and wet underfloor systems from electric underfloor heating.
Window Energy Rating
A certified A to G energy band for a whole window or doorset, issued under a quality-assured certification scheme. Approved Document L Table 4.2 allows a window to meet the standard by U-value 1.4 W/m2K or Window Energy Rating Band B, and building control may accept the certificate as evidence (para 10.4). Ask the glazing supplier for the certificate when specifying.

Pages where Part L comes up

27 guides reference Part L, grouped by build phase.

Want the full picture?

The buildwiz build guides walk through a UK extension in sequence and link to Part L (and every other Approved Document) at the exact stage it bites. That is usually a more productive way to use this material than reading the document cover to cover.

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