How many circuits does an extension need?

A typical extension needs 2–4 circuits: one lighting circuit (1.5mm² cable, 6A MCB) and one ring final for sockets (2.5mm² cable, 32A MCB). Add a dedicated circuit for cookers (6mm², 32A) or electric showers (10mm², 40A/50A). Each circuit needs its own MCB or RCBO in the consumer unit.

Do I need a new consumer unit for an extension?

Not necessarily. If your existing consumer unit has enough spare ways for the new circuits, you can connect directly. If not, you may need a new consumer unit or a sub-board (small secondary consumer unit) in the extension. All new consumer units must have metal enclosures per Amendment 3 of BS 7671.

What cable is needed for a detached outbuilding?

Detached buildings require SWA (steel wire armoured) cable buried at minimum 450mm depth (600mm under driveways). The cable size depends on the load and run length — typically 16mm² 3-core SWA for domestic loads. An earth rod and separate earth terminal may be required at the outbuilding.

Is extension electrical work notifiable under Part P?

Yes. New circuits in an extension are notifiable work under Part P of the Building Regulations. You must either use a registered competent person (e.g. NICEIC, NAPIT) who can self-certify, or notify Building Control before starting work and have it inspected. Non-compliance can cause problems when selling the property.

How many sockets should an extension have?

There is no minimum number specified in regulations, but good practice is one double socket per 4m² of floor area, or at least 2 double sockets per room. Kitchens need more — typically 6–8 double sockets plus dedicated appliance connections. It is much cheaper to install extra sockets during the build than to add them later.

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Extension Electrical Calculator — Circuits, Cable Sizing & MCBs

Plan extension electrical circuits including lighting, ring final, cooker and shower circuits with cable sizing, MCB selection and consumer unit spare ways check.

Extension type

Floor area

Number of rooms

Lighting circuit

Ring final (sockets)

Cooker circuit

How We Calculate This

This calculator determines the electrical circuit requirements for an extension based on the rooms, appliances and building type.

Circuit planning

Each circuit type has a specified cable size and MCB/RCBO rating per BS 7671 (IET Wiring Regulations). The calculator checks whether the existing consumer unit has enough spare ways.

Detached buildings

Detached outbuildings require SWA cable with buried installation. Cable sizing accounts for load and voltage drop over the run length.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.