Plan extension electrics before first fix: agree socket and lighting positions with your electrician, confirm the consumer unit has capacity (or budget for an upgrade), and ensure notifiable work will be certified under Part P by a registered installer. Electrical design should run in parallel with architectural drawings — not after walls are closed.
Extensions add new circuits for sockets, lighting, heating controls, and sometimes cooker or shower supplies. Getting the layout wrong is expensive to fix once plasterboard is up.
Why Electrical Planning Cannot Wait
Builders need to know where to leave:
- Cable routes and draw points for downlights
- Back boxes at the correct heights
- Routes from the extension to the existing consumer unit
- Any structural penetrations for cable entry
If you defer electrics until the extension is weathertight, you may face surface trunking, lost socket positions, or insufficient capacity for kitchen or living areas.
Part P of the Building Regulations applies to new circuits and work in special locations in England and Wales (Electrical Safety First). Extension wiring is notifiable. Your electrician should coordinate certification with the wider building project.
Circuits You Will Likely Need
A single-storey rear extension often requires at least:
- Socket circuit — ring or radial depending on load and layout
- Lighting circuit — ceiling points, pendants, or downlights
- Dedicated circuits — if the extension includes a kitchen area: cooker, hob, oven, microwave, fridge, dishwasher points as designed
- Heating controls — thermostats, underfloor heating manifolds, or radiator valves with electrical actuators
- Smoke detection — mains-linked alarms where building regulations require them in the extended property
- Data/TV — if structured cabling is part of your spec (low voltage, but plan routes early)
Two-storey extensions multiply socket and lighting counts. Loft conversions add similar planning for bedrooms and en-suites — bathrooms need zone-compliant accessories.
Our kitchen electrics renovation guide goes deeper when the extension is primarily a kitchen.
Consumer Unit Capacity
Every new circuit needs a spare way (or a larger board). Before work starts, your electrician should assess:
- Spare ways on the existing consumer unit
- Whether the main switch and tails are rated for additional load
- Whether RCD/RCBO protection can be extended correctly
- Main earthing and bonding capacity
If the existing board is already full, obsolete, or lacks RCD protection, a consumer unit upgrade may be part of the extension project. Budget using published UK benchmark ranges — then obtain a property-specific quote.
High-load plans (induction hob, heat pump, EV supply elsewhere on the property) increase supply requirements. Discuss total load with your electrician early.
Socket and Switch Positions
Walk the empty shell with your electrician and mark:
- Sofa and TV positions
- Kitchen worktop runs — above worktop height per usual practice
- Desk and home-office corners
- Doorways for two-way switching
- Exterior doors for outside light switches
Building regulations in England include accessibility guidance on switch heights in new dwellings — your electrician and builder should align on mounting heights for new build elements. For socket counts by room type, see how many sockets when renovating.
Coordination With Other Trades
| Trade | Electrical coordination |
|---|---|
| Builder | First-fix chases, back boxes, ceiling draw-in |
| Plumber | Immersion, underfloor heating, shower pump supplies |
| Kitchen fitter | Appliance positions and isolator switches |
| HVAC | Boiler controls, MVHR, heat pump wiring |
| Building control | Part P certificate alongside other sign-offs |
A single programme sequence prevents the electrician returning multiple times for avoidable reopening of finishes.
Feeding the Extension From the Main House
Cable routes from the existing consumer unit to the extension must be protected, correctly sized, and installed in permitted zones. Buried cables in walls have depth and routing rules under BS 7671.
Where the extension is detached (garden room, garage), an underground SWA cable or protected overhead route may be needed — similar principles to garden electrics.
Testing and Certification
On completion you should receive:
- Electrical Installation Certificate covering new circuits
- Part P notification via your registered electrician
- Updated circuit chart at the consumer unit
- Alignment with any EICR on the original installation if the whole house was tested
Keep certificates with your building completion documents.
Building Regulations Beyond Part P
Extension projects touch Approved Documents for structure, fire, ventilation, and energy — not only electrics. Your builder or architect coordinates overall compliance; your electrician handles the fixed wiring element and certification.
Notify your electrician early if the extension includes:
- Mains-linked smoke and heat detection where building control requires it in the extended dwelling
- Mechanical ventilation with electrical controls
- Underfloor heating manifolds and thermostats
- Structured cabling for data, doorbells, or alarm pre-wiring
Early coordination avoids chasing walls twice.
Staging Work With an Occupied Home
If you remain in the property during extension work, agree power downtime windows with your electrician and builder. Temporary supplies for essential circuits may be possible while new circuits are commissioned.
First fix in the extension often proceeds while the existing house remains live — segregation and safe isolation are the electrician's responsibility. Do not assume extension wiring can be energised before inspection and testing are complete.
Common Mistakes Homeowners Make
- Fixing kitchen supplier layouts without electrical input — appliance positions drive circuit design
- Under-specifying socket count — see socket planning for renovations
- Assuming the builder's quote includes certification — confirm whether electrical first and second fix are in scope
- Omitting outdoor or garden feeds planned for a later phase — ducting is cheaper before paving
Cost Factors (Benchmarks Only)
Extension electrical costs depend on floor area, specification, and board upgrades — not a single national price. Useful references:
- New circuit — national guides often cite £250–£450 per dedicated circuit as a rough domestic benchmark
- Consumer unit upgrade — see our consumer unit cost guide
- Full extension first and second fix — itemised quotes after design drawings
RIGS does not publish fixed extension packages. Contact us with plans for a quotation.
Link to Wider Renovation
If the extension project exposes poor wiring in the original house, discuss whether partial or full rewire is safer while access is good.
If you are buying a property with a recent extension, verify certificates for the new work — see electrical checks when buying.
RIGS Electrical carries out domestic installation work including extensions for homeowners in Tring and surrounding areas. House rewiring and installations cover new circuits and board upgrades.
Frequently Asked Questions
Who designs extension electrics — architect or electrician? Architects set room layouts; a registered electrician designs circuits to BS 7671. Both should collaborate before first fix.
Is extension wiring notifiable under Part P? New circuits and work in special locations are notifiable in England and Wales. Your registered electrician self-certifies.
Can I use wireless sockets instead of fixed points? Not as a substitute for fixed socket provision in habitable rooms. Building regulations set minimum outlet requirements in new work.
How many circuits for a kitchen extension? Often multiple — ring for sockets, dedicated cooker/hob, appliances, lighting, and possibly extraction. Design depends on appliance spec.
Do I need building control for electrics if I have Part P? Registered electricians notify competent person schemes; building control still oversees the wider extension.
Should electrics be planned before planning permission? Conceptual load planning helps, but detailed design follows approved layouts.
*Building regulations and BS 7671 apply. This guide is general planning advice for homeowners — not a substitute for project-specific design by a registered electrician.*