Electrical requirements for a house extension

Start with the existing installation

An extension adds circuits to an installation that already exists, so the first question is what that installation can take. Spare ways in the board, adequacy of the existing protection, the earthing arrangement, the size of the main protective bonding and the condition of the existing circuits all determine whether new work can simply be added.

In older properties this is frequently where the budget surprise lives. Adding four new circuits to a full board with no RCD protection and undersized bonding means a board upgrade, and it is far better to know that at the drawing stage than the week the plasterer is booked.

• Count spare ways and check existing protection
• Verify main protective bonding size and continuity
• Consider an EICR on the existing installation before starting
• Budget for a board upgrade if the board is old or full

Designing what goes in the new space

Walk the room in your head at three points in the day. Where will you sit, where will the television go, where does the vacuum need to reach, where will a lamp live, where does a phone charge overnight. Then add half again to the socket count, because nobody in the history of extensions has said there were too many sockets.

Lighting should be planned as layers rather than a grid of downlights. General light, task light where you will read or work, and something low for the evening. If the extension is open plan to a kitchen, plan the switching so you are not lighting the whole space to make a cup of tea.

• Sockets: plan generously and think about furniture positions
• Separate lighting circuits or zones for large open plan spaces
• Data cabling to television and office positions while walls are open
• Outdoor socket and light on the new elevation, planned in not bolted on

Heating, ventilation and glazing

Underfloor heating, air conditioning, an air source heat pump or a bathroom in the new space all bring their own supply and control requirements. Underfloor heating in particular has to be coordinated with floor build up and screed timing, which means it is a first fix decision, not a fit out one.

Large glazed extensions get hot in summer and lose heat in winter, so cooling or additional heating is worth deciding early. Retrofitting a supply for either after the floor has gone down is expensive in a way that pouring a cable into an open floor is not.

Sequencing with the builder

The electrician needs two clear visits and one earlier conversation. The conversation happens before the build starts, so the design and any board work are agreed. First fix happens once the structure is weathertight and before plastering, with all cables run, back boxes fixed and positions agreed on site with a pencil on the wall rather than off a plan.

Second fix happens after plastering and decoration, when accessories and fittings go on and everything is tested. The classic delay is a client choosing light fittings during second fix week. Choose them at first fix, because a pendant, a downlight and a track all want different things behind the plaster.

Notification, inspection and paperwork

The extension itself is building control work, and the electrical installation within it is notifiable under Part P. Where the electrician is registered with a competent person scheme, the electrical work is self certified and notified, and the building inspector will expect to see that certification as part of signing off the extension.

You should receive an Electrical Installation Certificate with the full schedule of test results, plus the building regulations compliance notice. Do not let the job finish without them: the completion certificate for the whole extension can hang on that paperwork, and so can the sale of the house in five years.