House Extension and Electrical Work

Adding a new room to a UK house brings electrical work into scope automatically — the new space needs its own circuits or extensions of existing ones, and the work is notifiable under Part P of the Building Regulations. Typical electrical cost on a single-storey rear extension runs £2,500–£6,500 on top of the build cost itself; for a two-storey extension or a loft conversion, expect £4,000–£9,000.

The electrician should be involved from first-fix — running cables before plaster — through second-fix (sockets, switches, lights) and on to final test and Part P certification. Trying to retrofit electrics after plastering is an expensive mistake.

What's typically in scope

• Lighting circuit extension — new pendants, downlights or LED panels, switched from sensible locations. £150–£300 per light point.
• Sockets and dedicated circuits — typically a new ring main if the extension is over 25 m², plus dedicated circuits for kitchen appliances if it's an open-plan kitchen-diner.
• Consumer unit upgrade — older consumer units (rewireable fuse box, or split-load without RCDs) need replacing to a modern 18th edition unit. £450–£900 supplied and fitted.
• Smoke and heat detectors — Building Regs require interconnected mains-powered alarms in any extended dwelling. £350–£700 for a typical two-bed extension.
• Outdoor power and lighting — security lights, garden sockets, EV charger feeds. From £150 for a basic outdoor light to £900+ for an EV charge point.

Typical UK costs

Estimates assume an experienced extension electrician working with the builder; main-dealer or premium fit-out specialists charge 30–50% more.

Part P and Building Regulations

Any new circuit, consumer unit replacement, or work in special locations (kitchens, bathrooms, gardens) is notifiable. There are two paths:

• Use a Part P registered electrician (NICEIC, NAPIT, ELECSA scheme members) who self-certifies and arranges the certificate via their scheme. Easier and cheaper — most extension electricians are scheme members.
• Notify Building Control directly and have them inspect, with a non-registered electrician doing the work. Adds £300–£600 in inspection fees and slows things down.

The Part P certificate is essential when you sell — solicitors will ask for it and an absence creates a price-reducing question on the sale.

First-fix, second-fix, final test

Three distinct stages with different timing requirements:

• First-fix — runs cables before plaster, sets back boxes, drops feeds for sockets and switches. Has to happen after plumbing first-fix and before plastering. 3–5 days for a typical extension.
• Second-fix — installs sockets, switches, light fittings, smoke alarms after plastering and decoration. 2–3 days.
• Final test and certificate — half a day of testing, paperwork, and Part P notification.

Things people often miss

• Existing consumer unit capacity — older split-load CUs may not have spare ways for new circuits. A unit upgrade often becomes mandatory and should be priced from day one.
• Earthing arrangement — some older houses have TT earthing (rod-based); modern installations almost always require TN-S or TN-C-S. Upgrading the earthing arrangement adds £250–£600 and may need DNO involvement.
• Cable routes — running cables from the new extension back to the consumer unit can mean lifting flooring or chasing walls in the existing house. Plan route choices early to minimise disruption.
• Smart-home and CAT6 — first-fix is the cheapest moment to run data cables, smart-switch loops, and electric blind feeds. Adding them later means lifting newly-laid floors. Decide spec before the plaster goes up.
• EV charger — even if you're not buying an EV now, dropping a 16 mm² cable on the first-fix budget for £300–£500 saves a £1,500 retrofit later.

Frequently asked questions

Do I need a separate electrician or can my builder handle it?

Most builders will subcontract to a Part P registered electrician — they don't usually self-perform electrical work. Either source your own (often slightly cheaper) or let the builder include it in their package. Either way, the electrician must be Part P registered for the certificate.

Will my consumer unit need replacing?

Probably yes, if it's pre-2008 (rewireable fuses, or a split-load without RCDs in every circuit). Modern Building Regs and the 18th Edition Wiring Regs require RCD or AFDD protection on most circuits. Budget £450–£900 supplied and fitted.

How long does the electrical work add to an extension build?

Roughly 1 week first-fix + 2–3 days second-fix + half a day for testing. Total ~7–10 working days spread across the build, fitted around plumbing, plastering and decoration.

Do I need a new ring main for an extension?

For extensions under 20 m², extending the existing ring main is usually fine. For larger extensions or open-plan kitchen-diners with multiple appliances, a new ring main avoids overloading the original. Your electrician will assess based on layout and load.

What happens if Part P isn't certified?

Building Control can refuse completion certificate, and your buyer's solicitor will flag it as a defect on sale. Retrospective certification costs £400–£900 and means an EICR-style inspection plus any remedial work — much more expensive than doing it right the first time.

Can I run my own data cables alongside the electrician's first-fix?

Possible if you're comfortable with low-voltage data work, but coordinating with the electrician to run them in the same channels saves both time and money. Most extension electricians will run CAT6 alongside power cables for £30–£60 per data point.

Want a local pro to handle this? A Part P registered electrician with extension experience is essential. Don't try to add electrical work to a builder who isn't electrically qualified — Part P certification is non-negotiable for resale.