Garden room vs extension: which costs less?

When you need more space at home, the two options most people consider are a house extension or a garden room. Both give you extra square metres. Both can be used for similar things: an office, a living area, a playroom, a gym. But the cost, the disruption, and the timeline are very different.

We're not going to pretend we're unbiased here. We build garden rooms, not extensions. But we'll give you the honest numbers so you can make a fair comparison. Some situations genuinely call for an extension, and we'll say so.

A single-storey rear extension in the North West typically costs between £1,500 and £2,500 per square metre, depending on the spec and the builder. That's the build cost alone, before you add in architect fees, structural engineer fees, planning application fees, building control fees, and any landscaping to put the garden back together afterwards.

For a 16-square-metre single-storey extension (roughly 4m x 4m), you're looking at a realistic all-in cost of somewhere between £35,000 and £55,000. That includes foundations, building work, roof, windows, plastering, electrics, plumbing, and decoration. A higher spec finish with underfloor heating, bi-fold doors, and quality flooring pushes it towards the upper end.

A two-storey extension costs more again, typically £1,800 to £2,800 per square metre, but you get double the floor space for less than double the cost because the foundations and roof are shared.

These numbers are based on real costs in Cheshire and the North West in 2026. London and the South East are higher. Rural areas can sometimes be lower if you find the right builder.

A properly built, fully insulated garden room from us costs between £12,500 and £42,000 depending on the size and type. That price includes everything: the base, the frame, insulation, cladding, roofing, windows, doors, full electrical package, internal lining, delivery, and full installation.

For a direct comparison with that 16-square-metre extension, a 4x4m garden room from us costs between £17,500 and £21,000 depending on the model. That's fully installed, with no hidden extras.

The price difference is significant. A garden room delivering the same usable floor area costs roughly half what an extension costs, sometimes less. And the price we quote is fixed. There are no surprise extras, no architect fees, no planning fees, and no building control charges.

This is where the gap gets even wider.

A single-storey extension typically takes 8 to 14 weeks from start to finish. That includes digging foundations, building the walls, fitting the roof, first and second fix electrics and plumbing, plastering, decorating, and snagging. During that time, you're living in a building site. There's dust, noise, tradespeople in and out, and often a missing wall or compromised kitchen for several weeks.

A garden room takes 5 to 7 working days. We arrive, we build, and we leave. You don't lose any part of your house during the process. There's no dust in the kitchen, no plastic sheeting over the furniture, and no coming home to find the back wall missing.

For families with children, people working from home, or anyone who just values their sanity, that difference in disruption is worth considering. A week of building at the end of the garden is a very different experience to three months of construction work attached to your house.

Most single-storey rear extensions up to 6 metres (or 8 metres for detached houses) can be built under permitted development, though you need to go through a prior approval process with the council, which takes 42 days.

Most garden rooms don't need planning permission at all if they meet the permitted development rules we've described in our planning guide. That's one less thing to deal with and one less delay in the process.

Extensions that go beyond permitted development limits need a full planning application, which can take 8 to 13 weeks and isn't guaranteed to be approved. That's a risk and a delay that doesn't apply to most garden rooms.

This is the one area where an extension has a genuine advantage. Because an extension is part of the house, it can increase your official living space and the number of bedrooms. That directly affects your house's value and how estate agents describe it.

A garden room is a separate building and isn't counted as part of the house's internal floor area on an estate agent's listing. It adds value, but not in the same way as an extra bedroom or a larger kitchen.

If what you need is another bedroom, a bigger kitchen, or a larger bathroom, an extension is the right choice. A garden room can't give you those things.

But if what you need is a home office, a gym, a studio, a playroom, an entertaining space, or just somewhere to go that isn't the house, a garden room does that job for less money, less disruption, and in a fraction of the time.

Both a well-built extension and a well-built garden room add value to your property. The exact amount depends on your area, the quality of the build, and what the room is used for.

Estate agents generally estimate that a single-storey extension adds between 5 and 10 per cent to a property's value. The return depends heavily on the cost of the extension and the value of the house. Spending £50,000 on an extension to a £250,000 house is a different calculation to spending the same on a £600,000 house.

Garden rooms are increasingly listed as a feature in property listings, and agents report that they appeal to buyers, especially those who work from home. A properly built, insulated garden room in good condition is an asset. The return on investment tends to be better than an extension simply because the initial outlay is lower.

An insulated garden room is cheap to run. The 50mm Kingspan insulation we use means the room heats up quickly and holds its temperature well. Most of our customers run a small electric heater or fan heater in winter and find it costs a few pounds a week at most. If you add underfloor heating, it's more comfortable but slightly more to run.

An extension is part of the house, so it's heated by your existing central heating system. That can be more efficient if you already have a boiler running, but it also means the extension adds to your overall heating demand. The difference in running costs is usually small either way.

Garden rooms are subject to VAT at 20 per cent, and our prices include VAT. House extensions are also subject to VAT on labour and materials. Neither option has a VAT advantage over the other for most homeowners.

If you're building an annexe for an elderly or disabled relative, there may be VAT relief available under certain conditions. Ask your accountant about this if it applies to your situation.

Here's a straightforward side-by-side for a 16-square-metre space:

If you need to increase the internal living space of your house, such as adding a bedroom, enlarging a kitchen, or creating a bigger living room, an extension is the way to go. It costs more and takes longer, but it changes the house itself.

If you want additional usable space for working, exercising, creating, or relaxing, and you don't want to spend months in a building site or tens of thousands of pounds more than you need to, a garden room is the better option. It gives you a properly built, insulated, year-round space for roughly half the price and a tenth of the build time.

We'll always be straight with you about which option makes sense for your situation. If you're not sure, get in touch and we'll talk it through. No pressure, no sales pitch, just a fair and honest conversation about what works best for you.