AFDDs (Arc Fault Detection Devices): Do You Need One for Your Extension?

Your electrician hands you a quote with a line you don't recognise: "AFDD per circuit", at around £100 when installed. Six circuits, and the total climbs to a figure you weren't expecting. You ask what it is, get a one-word answer ("fire safety"), and now you're deciding whether to pay for something you don't understand or refuse it and worry you've cut a corner that matters. That decision is easier once you know what an arc fault is, and when the wiring regulations make it mandatory versus optional.

What an AFDD is and what it protects against

AFDD stands for Arc Fault Detection Device. It's a small protective module that sits inside your consumer unit (the metal box that used to be called a fuse box) and watches the electrical waveform on one circuit. When it spots the signature of a dangerous arc, it cuts the power to that circuit before the heat from the arc can start a fire.

To understand why that matters, you need to understand what an arc fault is, because it's a different problem from the ones your existing protection already covers.

A circuit breaker (an MCB, the switch that trips when you overload a socket) protects against too much current flowing. An RCD or RCBO protects against current leaking to earth, the fault that can electrocute you. Both of those involve a clear, measurable change in current that the device can react to.

An arc fault is sneakier. Picture a cable that's been pinched under a tight staple, nicked by a careless screw, or worked loose in a terminal over years of thermal expansion. Electricity jumps the tiny gap as a spark, an arc, and that arc burns hot enough to ignite the cable insulation, the timber it's stapled to, or the dust around it. The catch is that the current flowing through that arc can be low, well within the normal range for the circuit. The MCB sees a normal load and never trips. The RCD sees no leakage to earth and never trips. The arc just keeps burning. That's the gap an AFDD fills: it recognises the high-frequency electrical noise an arc produces and disconnects before it becomes a fire.

There are two kinds of arc fault. A series arc happens in a single conductor, like a loose connection in a socket terminal. A parallel arc happens between two conductors, like damaged twin-and-earth cable where the live and neutral are arcing across a damaged section. AFDDs are designed to catch both, and the series arc in particular is the one no other device in your consumer unit can see.

The relevant standard is BS 7671, the UK wiring regulations (the "18th Edition"). Amendment 2, published in 2022, is where AFDDs moved from a vague recommendation to a clear set of rules about where they're required.

When you actually need one

This is the question most homeowners are really asking, and the answer depends entirely on what kind of building you're working on.

Under BS 7671 Amendment 2, AFDDs are mandatory for socket circuits (up to 32A) in a few specific premises:

• Higher-risk residential buildings, meaning buildings with floors at 11m or more above ground level (high-rise flats)
• Houses in multiple occupation (HMOs), where unrelated people rent rooms in a shared house
• Purpose-built student accommodation
• Care homes

For a standard single-family house, including most kitchen extensions, single-storey rear extensions, and loft conversions, AFDDs are recommended but not mandatory. The regulations use the word "recommended", which in regulatory language means it's left to the designer (your electrician) to decide based on a risk assessment, but it isn't a hard requirement you'll fail an inspection for omitting.

So if you're extending a normal family home, you can legally have the work signed off without a single AFDD. Whether you should fit them anyway is a value judgement, and it's worth understanding both sides before you decide.

11m

BS 7671 Amendment 2 makes AFDDs mandatory for socket circuits in buildings with a floor 11m or more above ground. Below that height, in a normal house, they are recommended, not required.

The case for fitting them: arc faults are a real cause of electrical fires, and they're exactly the failure mode that decades-old protection can't catch. If your extension circuits run through timber-framed walls, insulated stud partitions, or under a new loft floor where a fault could smoulder unseen, the added protection has genuine merit. New build is also the cheapest time to fit them, because the slot in the board is being filled anyway.

The case against: at the prices UK electricians are quoting, the cost adds up fast across six or eight circuits, and the technology has a history of nuisance tripping, where the device disconnects a circuit because it misreads the noise from things like vacuum cleaners, power tools, or dimmer switches as an arc. Early units were notably twitchy. Current models are much better, but the reputation lingers, and plenty of experienced electricians remain sceptical of the cost-to-benefit ratio in a low-risk domestic setting.

There's a middle path that a lot of electricians take: fit AFDDs on the circuits where the risk is highest (bedrooms, where fires while you're asleep are most dangerous, and circuits buried in timber) and skip them on circuits where the cable is accessible and the risk is low. Ask your electrician what they'd recommend and why, and ask specifically which circuits they'd protect rather than accepting an all-or-nothing answer.

How they fit into the consumer unit

AFDDs are modular DIN-rail devices. DIN rail is the standard metal mounting strip inside every modern consumer unit that all the breakers clip onto, so an AFDD physically installs the same way every other module does: it clips on, gets wired in, and takes up width on the board.

The detail that matters for planning and cost is how much width each circuit needs.

A combined AFDD+RCBO does three jobs in one module: arc fault detection, earth-fault protection (the RCD function), and overcurrent protection (the MCB function). One slot per circuit. Cleaner, and it keeps the board compact, but the modules are more expensive.

A separate AFDD plus a separate RCBO means two modules per circuit: one for the arc protection, one for the earth-fault and overcurrent protection. Two slots per circuit. Cheaper per module, but it eats board space fast, and a six-circuit extension protected this way needs twelve slots just for those circuits.

This is why board sizing matters before you commit. If you're adding AFDDs to several circuits, you may need a bigger consumer unit, or a board with more ways than you'd otherwise have specified. Sort this out with your electrician at the design stage, not when they open the box on site and find they're three slots short.

What to buy and what it costs

You don't buy AFDDs yourself in most cases. Your electrician supplies and fits them as part of the first-fix electrical package, because the device has to match the consumer unit's manufacturer and chassis. You can't mix a Hager module into a Wylex board, for example. The brand of your board dictates the brand of your AFDD.

The names you'll see on UK boards are Hager, Wylex, Siemens (the 5SM6 range), ABB (the DS201 range), Schneider, and MK. All make AFDD and combined AFDD+RCBO modules. There's no homeowner-grade "budget brand" worth chasing here; you buy whatever matches your board, and the price difference between manufacturers is modest compared with the labour and the board itself.

The device cost on its own sits around £90-£105 per module at trade. By the time it's installed, with the electrician's labour and the part marked up, the figure most UK electricians quote works out at roughly £100 per circuit.

Multiply that across your circuits and the number gets real. Six circuits adds up quickly on top of the rest of the electrical work. That's the figure that makes homeowners pause, and it's a fair thing to pause over given they're optional in a standard house.

External resource

IET Wiring Regulations (BS 7671)

The Institution of Engineering and Technology publishes the wiring regulations and guidance notes that define exactly where AFDDs are mandatory.

electrical.theiet.org

Alternatives and the honest comparison

There's no like-for-like alternative to an AFDD, because nothing else catches a series arc. The real comparison is between protection levels.

The takeaway: an RCBO is the modern baseline for a new extension and gives you overload plus earth-fault protection in one slot. Adding arc protection means either a second module alongside each RCBO, or upgrading to the combined module. If board space is tight, the combined unit is the better buy despite the higher per-module cost, because two slots per circuit fills a board quickly.

If you decide AFDDs aren't worth it for your project, you're not leaving the circuit unprotected. A properly specified RCBO board is fully compliant for a standard house. You're declining the extra layer, not skipping a requirement.

Where you'll need this

• First fix electrics - the stage where the consumer unit is specified and the AFDD-versus-RCBO decision gets made
• Electrical layout planning - work out your circuit count here, because it drives how many protective modules and how big a board you need

Consumer-unit protection is a decision point on any project that adds new circuits, whether that's an extension, a loft conversion, or a rewire. The links above point to kitchen extension tasks, but the same protection logic applies across any extension or renovation that touches the electrics.

Common mistakes

Accepting the AFDD line on a quote without asking whether your building actually requires them is the first one. In a standard house they're optional, so you're entitled to a conversation about value before you pay for six or eight of them.

The opposite mistake is refusing them on a building where they're mandatory. If your project is in a block with a floor 11m or more up, or the property is an HMO, AFDDs on the socket circuits aren't negotiable, and an electrician who skips them is signing off non-compliant work.

The last one is assuming more protection is always better regardless of cost. On low-risk circuits with accessible cabling, the marginal safety gain from an AFDD is small. Spending the money where the risk is real (concealed cabling, bedroom circuits) and skipping it where it isn't is a defensible, common approach. Blanket-fitting every circuit isn't wrong, but it isn't the only right answer either.