Everything you need to know about wiring a granny annex — separate supply vs sub-main, consumer unit design, kitchen and bathroom circuits, heating, fire separation, Building Control approval, and realistic 2026 pricing from £3,000 to £8,000.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
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Granny annexes — self-contained living spaces built in the garden or attached to the main house — are one of the fastest-growing areas of domestic construction in the UK. They provide independent accommodation for elderly relatives, adult children, or rental income, and they require a complete electrical installation equivalent to a small flat or bungalow.
Unlike a shed or garage conversion, a granny annex typically includes a kitchen, bathroom, living area, and bedroom. This means the electrical installation must cover every aspect of domestic wiring: general power and lighting, cooking circuits, bathroom special location requirements, heating, hot water, smoke detection, and potentially a separate metered supply.
This guide covers the complete electrical installation process from supply design through to certification, with practical guidance on the decisions that affect cost and complexity.
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The most important design decision for an annex electrical installation is the supply arrangement. There are two options:
The annex is supplied via a sub-main cable from the house consumer unit or a dedicated switch-fuse adjacent to the meter. This is the simpler and cheaper option. It is appropriate when the annex is ancillary to the main house (used by a family member, not separately metered or billed). The sub-main cable must be sized for the annex maximum demand — typically 16.0mm² or 25.0mm² SWA for a one-bedroom annex. The annex electricity cost is included in the main house bill.
The annex has its own electricity meter and a separate connection from the Distribution Network Operator (DNO). This is required when the annex is a separate dwelling (for rental, sale, or independent occupancy). The DNO installs a new service cable, cut-out, and meter. The electrician provides the meter tails and consumer unit. Lead time is typically 6 to 12 weeks. Cost is £500 to £2,000+ depending on the distance from the existing network and any excavation required.
Check the planning conditions for the annex — they often specify whether the annex is ancillary (tied to the main house) or independent. This determines the supply arrangement, Council Tax liability, and whether a separate meter is required.
The annex requires its own consumer unit with full RCD protection. A typical one-bedroom annex needs 8 to 12 ways:
An RCBO board is the recommended approach — it provides individual RCD and overcurrent protection for each circuit without the risk of one fault tripping unrelated circuits. This is particularly important in a self-contained dwelling where losing all power to a fault on one circuit affects an elderly or vulnerable occupant.
The annex kitchen requires the same electrical provision as a kitchen in a house:
The annex bathroom is a special location under BS 7671 Section 701. The key requirements are:
For an electric shower, the pull-cord switch must be outside Zone 1 (typically ceiling- mounted outside the shower area). The shower circuit requires a dedicated MCB/RCBO sized for the shower unit rating — typically 40A to 50A for a 9kW to 10.8kW electric shower.
Annexes need their own heating and hot water system. The electrical requirements depend on the system chosen:
Fire safety in an annex has direct implications for the electrical installation:
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The annex requires full Building Regulations approval. The electrical work is notifiable under Part P and must be either:
Building Control will want to see the Electrical Installation Certificate (EIC) as part of the overall sign-off. The EIC must cover all circuits in the annex and the sub-main (if applicable). Ensure the certificate is completed before the Building Control final inspection.
The annex installation must be fully tested and an EIC issued. The testing scope covers every circuit in the annex:
Annex electrical installation costs depend on the size, specification, and supply arrangement:
Annex electrical installations are the highest-value domestic job type after full house rewires. A typical one-bedroom annex is worth £4,000 to £6,000 for the electrical package alone, and the work spans 3 to 5 days over a 4 to 8 week build programme. These are premium customers building substantial extensions — they value quality, professionalism, and certification.
Price the complete annex electrical package with Elec-Mate's quoting app. Sub-main, consumer unit, all circuits, heating, testing, and certification — all itemised with your margins. A professional PDF quote wins the job against competitors who send a text message with a single number.
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