How to plan the electrics for a house extension

Helpful video reference. Master Builder Andy Stevens in association with Homebuilding and Renovating's series "Overview of Basic First Fix Electrics; How to Build an Extension" gives a clear overview of what first fix electrical work involves on an extension project. Homebuilding and Renovating is the UK's leading self-build and home improvement magazine.

1. Decide what you actually need before you start drawing

Think about how the room will actually be used, not how it looks on a plan. A kitchen extension needs a minimum of two ring final circuit sockets at worktop height, plus dedicated circuits for the oven and hob. A living room extension can typically run on the existing ring circuit if the additional load is modest and the cable length stays within limits. A utility room needs an outlet for the washing machine and potentially a separate feed for a tumble dryer.

Write a list of appliances, screens, chargers and lights before the electrician arrives. Adding sockets during first fix costs very little. Adding them after boarding costs considerably more.

2. Mark socket and lighting positions on the floor plan

Draw the room to rough scale and mark where furniture will go. Sockets behind a fixed sofa are useless. Mark your TV wall and put a double socket there. If the extension will have a dining area, consider floor-level sockets or positions that clear the dining table on both sides.

For lighting, decide whether you want ceiling pendants, downlights or a mix. LED downlights are popular but require fire hoods above them if they penetrate a ceiling between occupied floors — cost that in. Mark switch positions on the same side of the door as the handle, at a consistent height (typically 1,200 mm from floor level for switches, 450 mm for sockets, unless you are applying accessible standards).

3. Plan the cable routes before first fix begins

Cables from the consumer unit to the extension can follow one of two routes: surface run in trunking or conduit (visible but quick to change), or chased into the wall (invisible but permanent). For a new extension, chased cable is almost always the right call — the walls are being plastered anyway.

The cable run from the existing house into the extension usually passes through the cavity wall or along the wall plate. Agree the entry point with your builder before the brickwork goes up. Altering it later involves core drilling and waterproofing.

4. Understand the safe zones for concealed cables

BS 7671 requires cables to run in prescribed safe zones in walls: vertically from the accessory position, horizontally from the accessory, or within 150 mm of the ceiling. Outside these zones, cable must be protected by conduit or be at a depth of at least 50 mm. This matters because anyone drilling or nailing into the wall later will expect cables to follow these routes.

The electrician should photograph all cable runs before the plaster goes on — ask specifically for this. Those photos are worth keeping with the house paperwork.

5. Brief the electrician before they arrive

Share your socket and lighting layout with the electrician before the day they start. Ask them to confirm: whether the extension circuits will be added to the existing consumer unit or whether a sub-board is needed; whether the existing consumer unit has spare ways and adequate capacity; and what RCD protection will cover the new circuits.

Extensions in England require Part P notification if new circuits are added — the electrician should handle this and include it in their quote. Confirm it is included before work starts.

6. First fix: what the electrician does while the walls are open

During first fix the electrician fits the back boxes (socket and switch mounting boxes), runs all the cable from the consumer unit to each position, and terminates the cables at the board end. Nothing is connected to accessories yet — the outlets are open boxes with cable tails hanging out.

The key tasks are: mounting back boxes at the agreed heights, running cable in agreed routes, pulling cable through stud walls using fish tape, fixing cable clips at the correct spacing, and leaving enough tail at each position for the second fix connections. Inspect the finished first fix before the boarding or plastering goes on. It is your last chance to add a position cheaply.

7. Second fix and certification: what you receive at completion

Second fix happens after decoration. The electrician connects the accessories (sockets, switches, light fittings), makes connections at the consumer unit, and carries out the commissioning tests: continuity, insulation resistance, polarity, and earth fault loop impedance. These tests are required by BS 7671.

Once satisfied, the electrician issues an Electrical Installation Certificate (EIC) and a schedule of inspections and test results. You should also receive notification that the work has been registered with building control under Part P. Keep all three documents with the house deeds.

When to call us

Richard handles extension wiring for homeowners in Sandwich, Deal, Dover and east Kent. If you are in the planning stage and want advice on circuit design, cable sizing, or whether your existing consumer unit has room, a short call or visit costs nothing and saves expensive changes later.