What electrical work does a home extension need?

Why does an extension need new circuits rather than extending the old ones?

Because the existing circuits were designed for the house as it was. A ring final circuit serving the whole ground floor already has its share of kettles, dishwashers and heaters on it, and a kitchen extension often doubles that load. Adding a new room on to the end of it is how a circuit ends up permanently near its limit.

The usual shape for a single storey rear extension is a new socket circuit, a new lighting circuit, and a separate radial for each fixed appliance that draws heavily: an electric hob or range, a built in oven, electric underfloor heating, and increasingly an air conditioning unit or a heat pump outdoor unit. Bifold doors with integral blinds, a garden room feed and an outside socket tend to get added to the list once someone starts thinking about it.

There are cases where extending an existing circuit is sensible. A small side return that adds two sockets and a light to an under-used circuit is one. We will say so if that is your case, because a spur costs less than a circuit. It is just not the default anyone should assume.

Will my consumer unit cope, or do I need a new one?

This is the question that sets the shape of the job, and it has three honest answers.

• The board has spare ways and RCD protection. The new circuits go in on their own breakers and the board stays. This is the cheapest outcome and it is common in houses rewired or reboarded in the last fifteen years or so.
• The board is sound but full. A small second board next to it, fed from the main one, takes the extension circuits. Cheaper than a full replacement and perfectly acceptable when the existing board is in good order.
• The board is old, has no RCD protection, or fails on inspection. A new consumer unit is the right answer, and an extension is the least painful moment to do it because the house is already a building site.

What we will not do is tell you the board must go without looking at it. Plenty of extensions are sold a new board they did not need. An hour of inspection settles it, and if the answer is that the board is fine, that is what we say. We covered the repair or replace decision in more detail in our fuse board article.

The one thing a bigger board cannot fix is the main fuse. A large extension with an electric range, underfloor heating and a charge point can push a house towards the limit of its supply, and only the network operator can change that. It is rare, and it is worth knowing early rather than late.

When does the electrician come in, and how does it fit the builder’s programme?

Twice, at fixed points, and the builder’s programme has to allow for both.

First fix happens once the shell is up and before plasterboard or plaster goes on. Cables are run through the joists and studwork, back boxes are fixed for every socket, switch and outlet, and the feeds are brought back to the board position. This is also when anything that needs a cable in the wall goes in: TV and data points, speaker cable, the feed for an outside light, the supply for a future garden office. Once plaster is on, every one of those becomes a chase and a repair.

Second fix happens after the walls are finished and usually after decoration. The accessories go on, the lights are hung, the appliances are connected, and the circuits are tested and energised.

The point where extensions go wrong is between the two. If the builder plasters before the electrician has been, or the kitchen designer moves the island after first fix, the cost of that change is out of all proportion to the work. Our first and second fix for builders page explains how we sit inside a programme, and for a homeowner the practical advice is simple: have the socket and lighting plan agreed, on paper, before first fix day.

Who signs the electrical work off, and does building control need to see it?

The new circuits are notifiable work under Part P of the Building Regulations. That is not optional and it is not a formality: it means the work has to be certified and the local authority has to be told it was done.

There are two routes. Because we are an NICEIC Approved Contractor and a registered competent person, we self certify: we test the work, issue an Electrical Installation Certificate, and notify building control ourselves, and a compliance certificate follows in the post. The other route is for the electrical work to be inspected by building control as part of the extension application, which costs more and means waiting for the inspector.

The honest point here is the one we are asked about most. We cannot certify work we did not do. If a builder’s labourer runs the cables and we are asked to connect them up and sign for them, the answer is no, because the certificate says we designed, installed and tested the installation, and we would be putting our name to something we did not see. That is not us being difficult. It is the reason the certificate is worth having when you sell the house.

What changes the price of the electrical work in an extension?

We do not quote extension electrics from a description, because the things that move the price are the things you cannot see from one.

• The consumer unit decision above. Spare ways, a second board, or a new board.
• How far the extension is from the board, and what the cable route passes through. A rear extension on a house with the board at the front, and a solid floor between, is a longer run.
• The number of circuits, which is mostly driven by the kitchen. An all electric kitchen is three or four radials on its own.
• Underfloor heating, air conditioning, or a heat pump. Each is a dedicated circuit with its own isolation, and the heating mats need their own controls.
• External work. Outside sockets and lighting are cheap to run at first fix and expensive later.
• Whether the rest of the house needs anything doing at the same time. An extension often exposes the fact that the existing lighting circuit has no earth, or the bonding is missing, and it is far better to hear that at first fix than at the final test.

One thing that helps: give us the kitchen plan and the builder’s drawings before we quote. Half of the variation between quotes on the same extension is one electrician guessing and another one counting.

Questions we get asked about this