Garden Room Power Capacity: How Many Amps You Need

Most garden rooms that are used as an office or studio need a dedicated 32A supply. A simple cabin with lights and a couple of sockets can manage on 16A. The right answer depends on what you plug in, so the trick is to add up your appliances first, then choose the cable and consumer unit to match. We’ll walk you through it step by step, in plain English.

TL;DR

• Power capacity is the amount of electricity your garden room’s supply can safely deliver at once, measured in amps (A).
• Lights, a laptop and a few sockets: a 16A or 20A circuit is usually enough. A garden office with a heater: plan for 32A. Add an EV charger or hot tub and you need a bigger, dedicated supply.
• Work out amps with one sum: watts ÷ 230 = amps.
• The cable size depends on the load and the length of the run, so ask your electrician to confirm it.
• A new circuit to a garden room must be notified under Part P of the Building Regulations, or done by a registered electrician.

Table of Contents

• What does garden room power capacity actually mean?
• Step 1: List everything you’ll plug in
• Step 2: Turn watts into amps
• Step 3: Allow for everything running at once
• Which supply suits which cabin?
• Cable size: length matters as much as load
• Safety rules you can’t skip
• What does it cost?
• Plan the power before you choose the cabin
• A worked example
• Our view: plan for the life you’ll have in the cabin
• Ready to choose your cabin?
• FAQ
• Sources

What does garden room power capacity actually mean?

Think of it as the size of the “pipe” that brings electricity from your house to the cabin. A wider pipe lets you run more things at the same time without tripping the breaker or overheating the cable.

Capacity is measured in amps. Your home runs on 230V single-phase power, and every appliance pulls a certain number of watts. Divide the watts by 230 and you get the amps it draws. Add up everything that could be on at once and you know what your supply has to cope with.

For a typical garden office, electricians usually point to a 32A or 40A radial circuit, which most homes can support. That’s the starting point. Now let’s get specific about your own cabin.

Step 1: List everything you’ll plug in

Grab a pen and write down every item you expect to use in the cabin, now and in a few years. Be honest here. It’s far easier to plan for a heater or a second monitor today than to rip up a trench later.

Find the wattage on each appliance’s label or in its manual. Here’s an example list for a cabin used as a home office:

These are example figures. Always use the numbers printed on your own kit.

Step 2: Turn watts into amps

The sum is simple:

Amps = watts ÷ 230

Using the list above:

• Lights, laptop, heater and fridge come to 2,400W. That’s about 10.4A.
• Add the kettle and you’re at 5,400W, or about 23.5A.

So a cabin with a heater and a kettle running together already goes past what a 20A circuit should carry. That’s why a 32A supply is the usual choice for an office with proper heating.

A quick tip: the big hitters are anything that makes heat. Heaters, kettles, air conditioning and hot tubs use far more than lights and laptops. If you’re planning heating, our guide to calculating electric heating for your cabin shows how to size it for your floor area.

Step 3: Allow for everything running at once

It’s tempting to think you’ll never have the heater, kettle and computer all on together. In winter, you often will. Heating and IT kit run for hours, so count them close to their full rated load rather than assuming they’ll rarely overlap.

Electricians also use diversity allowances from BS 7671 (the UK wiring regulations) when they work out maximum demand. This is a professional calculation, and it can come out higher than a simple add-up. One homeowner’s cabin project showed a raw total under 5kW (21.7A) become a design demand of 36.65A once the allowances were applied, which led to a 40A breaker and 6mm² cable. Your own figure will differ, but it shows why you should leave the final sum to a qualified electrician.

As a rule of thumb, add some spare room. A consumer unit with spare ways and a supply with headroom costs little extra now and saves you a big job later.

Which supply suits which cabin?

Here’s a simple way to match your plans to a supply. Treat it as a guide, not a design.

If you plan to charge an electric car near the cabin, mention it early. A 7kW charger needs a dedicated 32A circuit and changes the cable and sub-board you’ll need.

Cable size: length matters as much as load

The cable between your house and the cabin is usually SWA (steel wire armoured) cable, which is tough enough to be buried in the ground. Its thickness depends on two things: how much power you’ll draw and how far it has to travel.

The longer the run, the more voltage is lost along the way. The wiring regulations limit that loss, and it’s one reason a long garden needs a thicker cable. As a rough guide, 6mm² three-core SWA suits many garden rooms up to about 30 metres with modest loads, while longer runs or heavier loads call for 10mm² or more.

Don’t pick a size from a table and hope. Your electrician will work out the exact voltage drop for your load and your route across the garden.

Burying the cable

SWA cable goes in a trench, and depth matters. We’ve put together a handy UK garden room cable depth checklist so you know what to plan before the digger arrives. Think about where the trench will run, what’s above it (lawn, path or patio) and whether anything could be dug up by accident later.

Safety rules you can’t skip

Garden electrics aren’t a place to cut corners. Here’s what the rules ask for.

• Proper wiring, not an extension lead. A cabin used as an office or studio needs a properly designed installation, with armoured cable in a trench. A lead trailing from the house isn’t safe or suitable.
• RCD protection. Outdoor circuits need 30mA RCD protection, and there should be a way to isolate the supply from inside the building.
• A separate consumer unit. A typical installation has its own consumer unit in the cabin, with spare capacity for later additions.
• Earthing. How the cabin is earthed depends on your home’s supply. Your electrician decides this, and often uses a local earth rod for a detached building.

Part P and certificates

Part P of the Building Regulations covers electrical work in homes, including gardens and outbuildings. Adding a new circuit has to be notified to Building Control, or carried out by someone registered with a Part P competent person scheme.

The simplest route is to use a registered electrician. Electrical Safety First recommends choosing someone registered with an approved scheme such as NICEIC, NAPIT or Blue Flame Certification. They work to BS 7671 and give you a certificate showing the installation was designed, built, inspected and tested to that standard. Keep that certificate safe. You’ll want it if you ever sell the house or claim on insurance.

What does it cost?

Costs vary with the distance, the trenching and how much kit goes in the cabin. One electrical firm’s 2026 guide puts a basic supply up to 15 metres at £600 to £1,000, a standard garden office up to 25 metres at £1,000 to £1,800, and a full specification over 25 metres at £1,800 to £3,000 or more. Trench digging can add £200 to £600.

Treat these as ballpark figures and get a written quote for your garden. Spending a little more on capacity now is usually cheaper than upgrading a supply that’s too small.

Plan the power before you choose the cabin

The best time to think about power is before you order. Your cabin’s layout decides where sockets, the consumer unit and the cable entry sit, and that makes the electrician’s job easier.

A few things to think about:

• Size and rooms. A bigger cabin, or one split into several rooms, usually means more lighting, more sockets and more heating. Our multi-room log cabins are a good place to see what layouts are possible.
• Use. An office needs different power from a relaxing summer house. Have a look at our garden summer houses if you want a lighter-use space, or our garden offices for full-time working.
• Shape. The roof style affects where cables can run inside the walls and ceiling. Apex roof cabins give you roof space to work with.
• Size limits. Height and position affect planning rules too, so it’s worth reading our guide on garden office sizes for UK homes.
• Why bother. If you’re still weighing it up, here are the key benefits of adding electrics to a garden room.

If you’re not sure what kind of cabin suits your plans, our guide on how to choose the right garden office takes you through it.

A worked example

Let’s say you want a 4m x 3m cabin as a home office, about 15 metres from the house. You’ll have a laptop, a monitor, LED lights, a 2kW heater and a small fridge. That’s 2,400W, or about 10.4A. You’d also like to boil a kettle now and then, which takes the peak to roughly 23.5A.

A 20A circuit would be tight. A 32A circuit gives you comfortable headroom, and a consumer unit with a few spare ways covers future additions like a printer or a second heater. Your electrician checks the cable size against the 15 metre run and the load, and confirms the earthing.

Now imagine you add a 7kW EV charger on the cabin wall. That needs its own 32A circuit, so the whole design changes. This is exactly why we say: tell your electrician everything you might want, before the trench is dug.

Our view: plan for the life you’ll have in the cabin

We talk to people about their garden cabins every day, and the happiest ones always did the same thing. They thought about how they’d use the space before they picked a size.

Power is part of that. A cabin that starts as a quiet reading room can turn into an office in a year. So we like to design with a little room to grow.

That’s where a bespoke build helps. Our bespoke log cabin service lets you shape the size, rooms, doors and windows around your plans, so the layout works for sockets, lighting and heating from the start. Our garden log cabins can also be adjusted to suit.

Tell us what you’d like to use the cabin for, and we’ll help you shape it around that.

Martin

Ready to choose your cabin?

Once you know what you’ll plug in, picking the cabin gets much easier. Browse our garden log cabins for ready-made ideas, or our garden pavilions if you want a fully enclosed space with a different look. If you’d like something made to your exact needs, with the right room for sockets and a consumer unit, our bespoke log cabin service is the place to start. Our professional installation service is listed on the same page.

We deliver across the UK mainland, and you can get in touch for a tailored quote any time.

FAQ

Is a 16A supply enough for a garden room?

For a small cabin with lighting and a couple of sockets, yes, a 16A or 20A circuit often does the job. If you’ll run a heater and office kit together, you’ll want 32A. Your electrician will confirm what suits your plans.

How many amps does a garden office need?

Most garden offices are planned around a 32A circuit. To check yours, add up the watts of everything that could be on at once and divide by 230. A 2kW heater alone is about 8.7A, and a 3kW kettle is about 13A.

Can I run power to my garden room myself?

A new circuit to a garden room is covered by Part P of the Building Regulations. It must be notified to Building Control or done by someone on a competent person scheme. Using a registered electrician is the easiest way, and you get a safety certificate at the end.

Can I just use an extension lead from the house?

No. A cabin used as an office or studio needs a properly designed supply, with armoured cable buried in a trench and RCD protection. An extension lead isn’t a safe or lasting answer.

How deep should the cable go?

Depth depends on what’s above the trench. Our garden room cable depth checklist sets out what to plan for, and your electrician will confirm it for your garden.

Sources

• Garden Room Electrics: Cable, Consumer Unit & Costs
• Garden cabin electrics
• Garden Room Electrical Installation: Wiring
• Outdoor Electrical Installations UK: Garden
• Electrical Safety (Part P guidance)
• Electrical Safety First: How safe is your home 2025

Recommended

• Why Add Electrics To Your Garden Room: Key Benefits
• Start At 0.5 M: UK Garden Room Cable Depth And Checklist
• Calculating Electric Heating For Your Cabin
• Calculating Electric Heating Needs For Your Garden Log Cabin

Note: This content was created using AI writing assistance and reviewed by our editorial team. Readers are encouraged to verify critical facts, professional advice, or time-sensitive information independently.