What cable size do I need for a granny flat?

Most granny flats need 63A-100A submains. For a basic annex (lights, sockets, small appliances) up to 25m away, 16mm² SWA on 63A is often sufficient. With electric shower, cooking, and heating, 25mm² SWA on 80A-100A is more appropriate. Always calculate maximum demand and voltage drop.

Does a granny annex need its own consumer unit?

Yes, self-contained annexes need their own consumer unit with RCD protection. This provides local isolation, proper circuit protection, and complies with BS 7671. The consumer unit is fed by a submain from the main house supply.

What size submain for an annex with electric heating?

Electric heating significantly increases demand. Storage heaters might add 6-12kW, panel heaters 3-6kW. Combined with shower (9kW), cooker (10kW), and general load, you may need 80A-100A submain. Calculate your specific loads - 25mm² or 35mm² SWA is typical.

Can I run the annex from the house consumer unit?

The submain originates from the house consumer unit (via a main switch or RCBO), but the annex must have its own consumer unit. You cannot simply extend house circuits to the annex. The annex consumer unit provides local protection and isolation.

Do I need separate metering for a granny flat?

Not required for family use, but recommended for rental annexes. Separate metering requires DNO involvement and potentially a new supply. Sub-metering (without DNO) can track usage for billing. Check legal requirements if renting out.

What about earthing for a separate building?

The annex earthing connects back to the main house earth via the SWA armour and a separate earth conductor. PME earthing arrangements have specific requirements for outbuildings. Your electrician must verify earthing is suitable - additional earth electrodes may be needed.

How do I calculate maximum demand for an annex?

List all fixed loads: shower (9kW), cooker (10kW), heating (6kW), water heater (3kW), plus socket allowance (2-3kW). Apply diversity: 100% largest load + 40% of others. Example: 9kW + 40%(10+6+3+3) = 9 + 8.8 = 17.8kW = 77A. Size submain accordingly.

Is Part P notification required for annex electrics?

Yes, installing a new consumer unit and circuits in an annex is notifiable under Part P. This applies whether the annex is new-build or conversion. Work must be done by a registered competent person or inspected by Building Control.

Should I allow capacity for EV charging?

Yes, future-proofing for a 7kW (32A) EV charger is wise. This adds significant load - if your calculated demand is 60A, adding 32A EV = 92A, suggesting 100A submain. It's much cheaper to install larger cable now than upgrade later.

What burial depth for SWA cable to an annex?

Minimum 450mm under gardens/grass, 600mm under driveways or areas with vehicular access. Use sand surround and cable warning tape. Avoid routes near other services. Document the cable route for future reference.

Back to Cable Sizing Calculator

Annex & Granny Flat Cable Sizing Calculator

Calculate the correct submain cable size for annexes, granny flats, and self-contained accommodation

Annex Cable Quick Facts

Typical annex submain: 63A-100A depending on facilities
SWA cable required for buried runs between buildings
Annex needs its own consumer unit with RCD protection
Consider electric heating, cooking, and future EV charging
Part P notification required for new consumer unit installation

Cable Size Calculator

BS 7671:2018+A2:2022 compliant

1. Load Current

2. Cable Length (meters)

3. Installation Method

Lighting circuit (3% voltage drop limit)

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Common Annex Installations

Basic annex (lighting, sockets, small appliances)

63A submain, 16mm² SWA for runs up to 25m

Annex with electric shower + cooking

63A-80A submain, 25mm² SWA for typical runs

Full annex with electric heating

80A-100A submain, 25mm² or 35mm² SWA

Annex above garage (short run)

63A submain, may use smaller cable for very short runs

Annex with future EV charger provision

Size for additional 32A - consider 100A submain minimum

Related Outbuilding & Submain Calculations

Annexes share similar requirements with garden offices and garage workshops. If your annex includes electric heating, see our storage heater guide. Planning EV charging? Check our EV charger calculator to factor into your submain sizing.

Annex Submain Cable Sizing

A submain cable supplies a distribution board (consumer unit) in a separate building from the main supply. For annexes, this means running SWA cable from the house consumer unit to a new consumer unit in the annex. The submain must be sized for the maximum demand of all circuits in the annex, with allowance for diversity and future additions.

Annex Electrical Installation Costs (2024)

Typical UK installation costs including submain, consumer unit, and circuits.

Installation Scenario	Materials	Labour	Total
Basic annex submain (<15m)	£400-700	£500-800	£900-1500
Standard annex submain (15-25m)	£600-1000	£600-900	£1200-1900
Long run annex submain (25m+)	£800-1400	£700-1100	£1500-2500
Annex consumer unit (6-10 way)	£200-400	£300-500	£500-900
Complete annex electrical fit-out	£1500-3000	£2000-4000	£3500-7000

Prices as of 2024. Costs vary by annex size, cable run, and specification.

Maximum Demand Calculation

Maximum demand estimates the peak electrical load an annex will draw simultaneously. It accounts for diversity - not all appliances run at once. Typical annex calculation: 100% of largest load (usually shower or heating) + 40% of remaining loads. A well-equipped annex might have 15-20kW connected load but 8-12kW maximum demand.

Diversity in Annex Calculations

Diversity recognises that not all annex appliances run simultaneously at full load. BS 7671 and IET guidance provide diversity allowances: 100% for largest appliance, reducing percentages for others. Without diversity, a typical annex might calculate at 40kW+ but actual maximum demand is usually 15-25kW.

PME Earthing Considerations

Properties with PME (Protective Multiple Earthing) have specific requirements for supplies to outbuildings. If the submain exceeds certain lengths or the annex has extraneous metalwork, additional earth electrodes may be required. Your electrician must assess PME implications for annex installations.

Frequently Asked Questions

Related Electrical Calculators

Garden Office

Similar outbuilding requirements

Storage Heaters

Electric heating for annexes

Voltage Drop Calculator

Essential for long submain runs

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