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Garden room sizes

How much space an office, gym or studio really needs, what each size costs, and where size changes the rules.

Updated 22 October 2026

A bright, empty room with a sliding door onto a garden

The short answer: as a rough guide, one person at a desk can manage in about 6m², and a desk plus a sofa or rowing machine needs around 12m². A treadmill needs about 9m² on its own once you allow its clearances. Size also decides which rules apply: in England and Wales, under 15m² inside, a garden room is normally exempt from building regulations, and between 15 and 30m² a single-storey room is exempt only if it's at least 1m from every boundary or built substantially from non-combustible materials.¹²

How much space each use needs

Use	Guide
One person working	The workplace minimum is 11 cubic metres per person in an empty room.³ At a 2.4m ceiling, that's about 4.6m² of floor before any furniture, so we'd treat 6m² as a sensible minimum once you add a desk and chair.
Rowing machine	Needs a space of 2.74m by 1.22m in use.⁴
Treadmill	One UK model is about 1.8m by 1m, with at least 2.4m clear behind and 0.6m each side.⁵ That's roughly 4.2m by 2.2m of floor.
Office plus gym, or office plus sofa	Add the two together, plus room to move between them.

The office figure is a benchmark for workplaces, not a rule for home offices.³ The gym figures are manufacturers' own. Check the manual for your equipment, and think about ceiling height if you'll lift overhead.

Before you choose, mark out the footprint in the garden with canes and look at it from inside and out. Which? also warns that a room that takes over a small garden may put off future buyers.⁶

What each size costs

Size	Mid-range price
6m² small office	£7,000 to £9,000
12m² medium room	£18,000 to £24,000
20m² large room	£40,000 to £50,000

MyBuilder price guide, updated October 2026.⁷

MyBuilder says cost per m² tends to drop slightly on bigger builds, but its own size prices work out at about £1,200 to £1,500 per m² at 6m² and £2,000 to £2,500 per m² at 20m²⁷, so don't assume a bigger room will be cheaper per m². Use our cost calculator for your size.

How size affects the rules

Building regulations measure inside. In England and Wales, the 15m² and 30m² limits use the floor area to the finished inside faces of the walls.⁸ Thick insulated walls mean the footprint on the ground is bigger.
Planning measures the ground covered. In England, all buildings, other than the original house, must cover no more than half the garden.⁹
Near a boundary, height is capped. Within 2m of a boundary, the building can be no more than 2.5m tall.⁹ After the floor and roof, that leaves noticeably less ceiling height inside.
On protected land, far from the house, size is capped. In National Landscapes (areas of outstanding natural beauty), national parks, the Broads and World Heritage Sites, buildings more than 20m from the house can cover no more than 10m² in total.⁹
No sleeping. The building regulations exemptions only apply if there's no sleeping accommodation.¹
Scotland and Northern Ireland differ. See our Scotland and Northern Ireland guides.

Our free planning report checks your postcode for protected land, and our planning guide covers every rule.

Getting the size right

Ask for both internal and external sizes. Some companies quote internal sizes and some external, and the difference is the wall thickness on every side, so it can be significant.
Plan the furniture first, then choose the room.
If you're close to 15m² inside, decide whether going over is worth needing building regulations approval, a 1m gap from every boundary, or non-combustible construction.

Questions people ask

What is the most popular garden room size?

There's no official figure. For one person working, about 6m² is a practical minimum, and 12m² gives room for a second use such as a sofa or gym kit. MyBuilder prices a 12m² room at £18,000 to £24,000.⁷

What is the biggest garden room without building regulations?

In England and Wales, under 15m² inside, with no sleeping accommodation, it's normally exempt. Up to 30m² is exempt if it's at least 1m from every boundary or built substantially from non-combustible materials.¹

Is the 15m² limit measured inside or outside?

Inside. Building regulations measure floor area to the finished inside faces of the walls.⁸

Related guides

A white timber garden building with French doors and a red roof

Garden room costsReal prices by size, with a calculator.

A garden path leading to a small building at the bottom of the garden

Building regulationsThe 15m² and 30m² rules.

A modern dark-clad cabin with large windows in long grass

Garden office podsSmall rooms for one person.

Sources

legislation.gov.uk, The Building Regulations 2010, Schedule 2.
Welsh Government, Building regulations: outbuildings.
Health and Safety Executive, How much space should each person have in a workplace?.
Concept2 UK (manufacturer figures), RowErg.
NordicTrack (manufacturer figures), NordicTrack EXP 10i user manual (UK).
Which?, May 2026, How to buy the best garden room.
MyBuilder, updated 21 Oct 2026, Garden room cost guide.
legislation.gov.uk, The Building Regulations 2010, regulation 2.
legislation.gov.uk, GPDO 2015, Schedule 2, Part 1, Class E.