When should I order windows for my extension?

Order at foundations stage, not when the walls reach lintel height. Aluminium frames carry an 8 to 12 week lead time, stretching to 16 weeks in peak season. Order late and the building sits open to the weather while you wait, which can knock weeks off the end of the project.

Are aluminium or uPVC windows better for an extension?

Aluminium has slimmer frames that let in more light, handles large panes and triple glazing, and needs no maintenance, but costs roughly a quarter more than uPVC. uPVC is a sound budget choice, especially on side elevations nobody sees, while aluminium suits feature windows at the rear.

What U-value do extension windows need to meet?

The maximum U-value for windows in a new extension under Part L 2021 is set in the approved document, confirmed for your project via the SAP assessment. Double glazing with low-E coatings and argon fill normally meets it without needing to go triple.

Do extension windows need an egress or escape window?

Yes. At least one window in each habitable room must serve as a fire escape, meeting minimum width, height, openable area, and sill height rules. Your architect should design for this, but confirm the window schedule meets it before you order.

How should the gap around a window frame be sealed?

Properly sealed new windows use three layers: low-expansion foam for insulation, compressible expanding tape for weathertightness, and silicone over a backing rod on the outside. Foam alone, or silicone without a backing rod, is the most common defect and fails within a couple of years.

Why don't FENSA certificates apply to extension windows?

FENSA and CERTASS only cover replacement windows in existing openings. New windows on your build are signed off through your building control application instead, so do not let a supplier charge extra for FENSA certification on new work.

Back to Loft Conversion

Phase 4 · Structure · Task 06 of 07

Windows and Doors: Ordering, Lead Times, and Installation

How to specify, order and install new windows and doors. Covers frame materials, glazing specs, the lead time trap, installation day checks and the sealing details.

12 min readUpdated 2026Premium

Timeline infographic showing the window ordering and installation process from specification through lead time to installation day and finishing, with key decision points and coordination milestones highlighted

Windows and doors are where your build stops being a weatherproof shell and starts to feel like part of the house. Three things decide how well they perform: the frame material you choose, the glazing specification, and how carefully the gap around each frame is sealed. On top of that, they have to satisfy several parts of the building regulations at once: Part L for thermal performance, Part F for ventilation, Part K for safety glazing, and Part B for fire escape. Get those three decisions right and the rest is detail.

One of them needs making long before the rest, though, simply because of how long the windows take to arrive. Aluminium frames carry a 8-12 weeks lead time from order to delivery, stretching to 16 weeks in peak season, which makes them one of the longest-lead items on the whole build. Order them at foundations stage, not when the walls reach window height. Leave it too late and the building sits open to the weather while you wait: rain gets in, timber swells, plaster can't start, and your plasterer moves to another job.

What this guide covers

01When to order
02Choosing frame material
03Glazing specification
04Choosing a supplier
05What the NHBC Standards say about installation
06Installation day
07Sealing and weatherproofing
08Lead flashing and the roofer
09Window reveals and timber packing
10Managing the window company after installation
11FENSA and CERTASS don't apply
12What this costs

Premium content — unlock with an Access Pass

This guide is part of the buildwiz.uk Access Pass. Get full access to every project guide, planning through to completion.

Get Access Pass

£49, paid once. 24 months access.

Already bought the Access Pass? Sign in

General guidance for homeowners in England (and Wales where we say so), not professional advice. Every site is different: check anything that affects your build with a qualified professional, your local council or building control. Terms

Frequently asked questions

Reference guides for this task

Free guides to the tools, materials and terms this step involves.

Windows and Doors: Ordering, Lead Times, and Installation

What this guide covers

Premium content — unlock with an Access Pass

Frequently asked questions

Reference guides for this task

Tools

Materials

Background reading