Garden room vs house extension — the honest comparison

The garden room vs extension conversation is one of the most common ones we have on introductory calls. Both add usable space to a house. Both cost roughly the same order of magnitude. Both take a couple of months of your life. But they solve genuinely different problems, and the honest answer to 'which one should I build?' depends on two questions we work through with every client.

This is the like-for-like comparison at two real footprints — a 15 m² brief (single-room extension or a substantial garden studio) and a 25 m² brief (a proper double-room extension or a larger garden-room-plus-outdoor-kitchen brief).

The two questions we ask

Before any of the numbers, the choice depends on the answer to two questions:

• — 01Does the new space need to connect internally to the house? An extra bedroom, a bigger kitchen, a downstairs shower — these need internal connection and rule out a garden room. An office, gym, studio, therapy room, hobby space or hangout room usually doesn't — a garden room is the right answer.
• — 02Do you want to sell in the next 3-5 years? Both add resale value, but the ratios are different. An extension adds £1.20-1.60 of resale value for every £1 spent (in the South East and prime South West). A garden room adds £0.80-1.10 for every £1 spent. If resale is imminent, the extension is a better financial move even if the space itself is worse.

If the answers are 'no internal connection needed' and 'not planning to sell soon', the garden room wins on almost every other metric. Below is why.

Cost per square metre — the honest numbers

Devon and South West averages, all-in (build, VAT, planning, foundations, finishes) as of Feb 2026:

• — 01Garden room, 15 m² usable — £16,500-19,500 all-in. Cost per m² £1,100-1,300.
• — 02Garden room, 25 m² usable — £26,000-32,000 all-in. Cost per m² £1,040-1,280.
• — 03House extension, 15 m² usable — £42,000-58,000 all-in. Cost per m² £2,800-3,870.
• — 04House extension, 25 m² usable — £62,000-85,000 all-in. Cost per m² £2,480-3,400.

The extension cost premium is real and it's structural — the building has to marry to the existing structure, comply with the same Building Regulations as new-build housing, connect to existing services, and integrate architecturally with an existing façade. All of that adds cost that a garden building simply doesn't inherit.

Planning — the process time

Both routes generally start with the same question: is planning permission needed? For an extension the answer is usually yes above 30 m² (or under some circumstances above 4 m at the rear). For a garden room the answer is usually no, provided the design stays inside permitted-development volumes.

• — 01Garden room, permitted development — no application needed. £0, 0 weeks.
• — 02Garden room, pre-application enquiry (AONB / conservation area) — £115, 3-5 weeks.
• — 03House extension, planning application — £258 householder fee, 8-13 weeks council decision timeline.
• — 04House extension inside a conservation area or on a listed building — £258 planning + £258 listed-building consent, 12-18 weeks.

Add to that a full architect's package for the extension (usually £3,500-8,000 depending on scope) and the paperwork cost of an extension is £4,000-10,000 before a spade goes in the ground. A garden room's paperwork cost, on our quote, is £0 — we handle the design in-house and the AONB pre-app if needed.

Timeline from decision to occupancy

• — 01Garden room — typical 12-16 weeks from deposit to handover. Build itself is 5-8 days on site.
• — 02House extension — typical 8-12 months from decision to handover. Build itself is 10-16 weeks on site.

The build-time delta matters because an extension is a live-through-it experience. Ten to sixteen weeks of contractors in the house, a torn-up rear elevation, dust travelling through every room, and often a temporary kitchen. A garden room build touches the house for two half-days (services connection) and otherwise happens entirely outside.

Resale — the honest breakdown

The single argument for an extension over a garden room, in almost every conversation, is resale. And it's a genuine argument — a good extension adds more resale value than a good garden room. But the picture is more nuanced than 'extensions always beat garden rooms' at resale:

• — 01In prime South Devon markets (Salcombe, Dartmouth, Kingsbridge waterfront) a substantial garden studio adds £15,000-25,000 to a house sale — a genuinely meaningful multiplier.
• — 02In secondary markets (Newton Abbot, Ivybridge, non-prime Torbay) the garden-room resale premium is closer to £6,000-12,000 — worth having but not transformational.
• — 03An extension in a listed or conservation-area property can be difficult and expensive to sell if the extension didn't have the proper permissions. This is worth flagging — a poorly-consented extension is a negative at resale, not a positive.

“The garden room usually wins on time, cost per square metre, and disruption. The extension usually wins on resale. Neither wins on 'is it the right answer for you' — that's about the brief.”

— Arden & Oak — design lead

When we tell clients to build the extension instead

We regularly recommend the extension over the garden room. Three scenarios where we'll say it clearly:

• — 01The brief needs internal connection (bedroom, kitchen, bathroom, playroom for young children).
• — 02The house is a small terrace or cottage where an extra reception room downstairs would materially improve daily living.
• — 03The client is planning to sell within 2-3 years and the resale delta matters more than the working experience of the space itself.

For any other brief — home office, gym, studio, therapy room, hobby space, teenagers' hangout, guest sleeping accommodation, garden entertaining space — the garden room is the more sensible answer nine times out of ten. Faster, cheaper per m², cleaner build, and often architecturally more distinctive.

For our fixed-price ranges and typical footprints see the Standard Prices hub. For the pricing conversation in more depth, see the how-much-does-a-garden-room-cost guide. Or book a consultation and we'll walk through your specific brief without commitment.