How to wire a garden room or summer house

Helpful video reference. Nick Bundy of N Bundy Electrical (Stafford, 113,000+ subscribers) walks through a complete garden room electrical installation — cable sizing, SWA burial, sub-consumer unit wiring, earthing arrangement and the tests he carries out before handing over. Nick is a NAPIT-registered electrician and his commentary gets the UK-specific details right throughout.

1. Plan the circuits

Before ordering materials, work out what the garden room will actually need. A simple summer house used for occasional working might want one lighting circuit and two or three single sockets. A home gym or workshop may need a 16 A circuit for a heater, a separate circuit for tools, and outdoor-rated lighting for the path up to the door.

Each load feeds into the supply cable size calculation. Write out your intended circuits, their ratings and approximate lengths before the electrician visits, so the design stage is quick and the quote is accurate.

2. Confirm Part P notification

A new circuit from the dwelling to a separate building is notifiable under Part P of the Building Regulations in England. This means the electrician must either:

• be registered with a competent person scheme such as NAPIT or NICEIC, in which case they self-certify and notify on your behalf, or
• notify your local authority building control before work starts, who will inspect and issue a completion certificate.

You must receive either a Building Regulations Compliance Certificate or a local authority completion certificate at the end. Keep it with the house deeds. Buyers and mortgage lenders ask for it.

3. Size and route the supply cable

Most domestic garden room supplies use 6 mm two-core steel wire armoured (SWA) cable. The steel wire armour protects the cable mechanically and also acts as the circuit protective conductor (earth). The exact size depends on the circuit length and load, but 6 mm handles most domestic situations comfortably.

The cable must be buried at least 600 mm deep under unprotected ground, or 500 mm under solid concrete with yellow warning tape above it at 150 mm depth. Mark the route on a sketch before you cover it over — you will need to know where it is if you ever dig the garden.

Cable entering and leaving the house and the garden room both need proper SWA gland terminations: the armour stripped back, the gland torqued correctly, and a continuity test done to confirm the armour is electrically connected at both ends.

4. Install the sub-consumer unit

Inside the garden room, fit a small consumer unit. A two-way or four-way unit is usually enough. Every circuit inside an outbuilding must have 30 mA RCD protection, so either use a split-load board with an RCD covering all ways, or fit RCBOs throughout.

The unit must be in a dry, accessible location. Fix it to the wall using the cable entries at the correct knockouts. Label every way before the board is energised.

5. Wire the lighting circuit

Run 1.5 mm twin and earth from the consumer unit to each lighting point. In a small garden room, a simple loop-in arrangement works well: the supply comes in at the first fitting, loops to the next, and a switch drop comes off the last joint. Fit appropriate fittings with an IP rating suited to the conditions — IP44 minimum if the room gets humid.

6. Wire the socket circuits

Socket circuits need 2.5 mm twin and earth, protected by a 32 A RCBO or MCB inside the RCD-protected board. The wiring arrangement is a simple radial circuit (not a ring) — run from the consumer unit to each socket in sequence.

Use weatherproof or IP-rated fittings for any sockets that could be near moisture. If the room is unheated in winter, condensation on the walls is worth planning for.

7. Establish the earthing arrangement

This is the step most guides skip, and it matters. In the majority of UK homes, the earthing system is PME (TN-C-S), where the neutral and earth are combined in the distribution network and separated at your meter. The conditions of supply for PME prohibit exporting that combined neutral-earth to a separate building in many circumstances, because a break in the supply neutral can put a dangerous voltage onto the earth.

For a garden room, the common solution is to install a local TT earth: a rod driven into the ground at the garden room, connected to the earth terminal of the sub-consumer unit, with an RCD at the top of the installation for automatic disconnection. Your electrician will confirm the correct arrangement after checking your main earthing system.

8. Test, certify and notify

Before the supply is energised, the electrician must carry out initial verification tests: insulation resistance on every circuit, earth continuity on the SWA armour, polarity, and earth fault loop impedance. These results go onto an Electrical Installation Certificate.

If Part P applies (it does for this type of job), the certificate or building control sign-off must be in your hands before the installation is put into service. File it carefully.

When to call us

Garden room wiring is straightforwardly within scope for Sandwich Electrical. We design the circuit, size the cable, carry out all the tests and issue the Electrical Installation Certificate. We can also notify building control if you prefer not to deal with it. Call or WhatsApp for a fixed-price quote.