Extension electrics, wired to your builder's programme.

What the single-storey extension price covers

The price in the table is for a single-storey extension wired from first fix through to the test, with the sockets and lighting points an ordinary room needs. It includes the certificate for Building Control at the end.

It assumes the consumer unit you already have can take the new circuits. When it can't, upgrading the consumer unit is quoted on its own, and the survey is where I find that out.

A two-storey extension, or one bigger than the standard job, is priced from the drawings and the free survey, like any job missing from the price list.

First fix electrics before plastering

First fix is the part that ends up buried. New cables run from the consumer unit through the joists and studwork to every socket, switch and light position, and the back boxes go in. Anything that will need a supply later, an outside light or an extractor fan, gets its cable now.

It happens once the roof is on and the walls are up, and before any plasterboard or plaster closes them in. After that, moving a socket means cutting into a finished wall, so first fix is when the positions need to be settled. Marking the sockets and lights on the builder's plan before I start saves a lot of changing on site.

The cable routes have to respect the structure. How deep a chase can be cut into a wall, and where holes and notches can go in a joist, come under Part A of the Building Regulations.

Second fix, testing and the certificate

Second fix waits until the plaster is dry and most of the painting is done. That's when the faceplates and light fittings are put up and the new circuits connected at the consumer unit. Each one is tested before it's put into service.

The test results go on an Electrical Installation Certificate, which I issue for the extension, ready for Building Control.

It's me at the survey, at first fix, at second fix and at the test, so the person who wired the walls is the one who signs the certificate.

Building Regulations for extension electrics

Extending a house counts as building work under the Building Regulations, and most extensions need approval, so Building Control is usually involved from the start. A conservatory or porch can be exempt from approval, but Approved Document P is clear that its electrics are still covered by Part P when they share the house's supply.

The new circuits in an extension are notifiable electrical work under Part P. Approved Document P sets out three ways to certify notifiable work.

An installer who belongs to a competent person scheme certifies the work through that scheme. An installer who does not can appoint a registered third-party certifier to inspect and test it. Failing either, Building Control is told before the work starts, decides how much inspection it needs and takes the installer's Electrical Installation Certificate into account before issuing its completion certificate.

The existing consumer unit and wiring

Only the new work in an extension has to meet the current regulations. Approved Document P says the rest of the installation needn't be upgraded unless the new work would make it less safe, or its condition means the new circuits couldn't be used safely.

So I look at what the extension is joining before I price it. The board needs spare ways and suitable protection for each new circuit, and the earthing and bonding have to be up to it. The supply has to cope with the extra load as well.

On an older installation a look isn't always enough, and an electrical condition report is how to find out what state it's in before anything is added to it.

New kitchens and smoke alarms in extensions

When an extension is built to take a new kitchen, it needs more than the standard job allows, usually its own cooker circuit and a long run of worktop sockets. If the old kitchen is being stripped out at the same time, the kitchen rewire and the extension can be wired together at first fix.

Approved Document B expects a fire alarm system as well when an extension adds a bedroom or another habitable room upstairs, or a ground-floor one with no door straight outside. Those alarms go in the hallways and on each landing, mains-powered, each with a battery or other backup.

Loft conversions

A loft conversion is an extension upwards. The wiring follows the same first fix and second fix pattern, but adding a storey brings fire safety points that a ground-floor extension doesn't have, so the electrics for a loft conversion are priced separately.