Begin with the purpose, not the tape measure

The most common mistake is buying the biggest cabin the plot will take, then discovering the layout doesn't suit the way you actually use it. The right size falls out of two things: how many people will be in there at once, and what has to live in it permanently. A single-person office with a desk, chair and shelving is comfortable at 2.4m x 2.4m; the same footprint shared as a family art room, with four people around a table, feels cramped within minutes.

Write down a short list before you look at anything: who uses it, how often, what stays in it, and whether anything bulky ever needs to come through the door. That list decides your minimum size far more reliably than a showroom does.

Measure the plot, not the gap

Walk the space with a tape measure and a notepad before you compare cabin sizes.

• Mark the available rectangle with pegs, canes or string. A space measured on the ground is a very different thing from one imagined out of the kitchen window.
• Leave a maintenance gap of 450–600mm on any side you'll need to paint, re-felt or clear gutters. Timber needs attention every few years, and a cabin wedged tight against a fence is a misery to look after.
• Allow for roof overhang and any decking, which can add 300–600mm to the footprint in each direction.
• Check what you can't move: drains and manhole covers, meter boxes, tree roots, an existing shed base, the swing of the back gate.
• Note where the sun falls at the times you'll actually use the cabin. A west-facing window is lovely in June and unusable on a hot July afternoon without blinds.

Typical sizes and what they hold

Most garden cabins sold in the UK sit between roughly 2m x 2m and 5m x 4m, with a sweet spot for a single-purpose room around 3m x 2.5m. As a rough guide:

• 2m x 2m (4 sq m): potting shed, bike store, or a one-person hobby space. Room for a bench, a stool and shelving — but no sofa.
• 3m x 2.5m (7.5 sq m): the classic home office or studio. Desk, chair, filing cabinet, and a small armchair by the window if you keep the layout disciplined.
• 3.5m x 3m (10.5 sq m): a comfortable dual-purpose room — office plus occasional guest bed, or a gym with a treadmill and floor space to stretch.
• 4m x 3m (12 sq m) and above: a garden room that genuinely seats four to six, a family craft room, or a treatment room with a couch and a waiting chair.
• 5m x 4m (20 sq m) plus: annexe-style spaces, studios with a kitchenette, home cinemas. At this size, foundations, insulation and building control become serious considerations.

The space a cabin really occupies

Sellers quote the external footprint. What you stand inside, between finished internal walls, can be 150–250mm narrower on each side once framing, insulation and internal cladding are accounted for — more on a heavily insulated garden room than a simple shed. Check whether the quoted size is external or internal before comparing prices; a 3m cabin with thick walls can leave you less usable floor than a cheaper 2.8m one.

Then add the invisible space:

• Door swing. An 800mm door needs a clear quarter circle inside, or you'll be shuffling furniture every time you walk in.
• Eaves height. A cabin with 1.8m eaves feels very different from one at 2.1m, even at the same footprint. If you'll be standing or fitting tall shelving, eaves height matters as much as floor area.
• Circulation. Adults need around 600mm to pass and 900mm to pass comfortably. Furniture around all four walls with 500mm in the middle will feel tight, however generous the square metres.

Orientation, access and the base

Where the cabin sits shapes what size works. A structure tucked into a corner has only one or two usable elevations for windows and doors, which limits your layout — put the door on the short side and you free up the long wall for a desk or sofa.

Think about the route in, too. A 4m cabin at the bottom of a lawn sounds fine until you're barrowing base materials down a 20m path or squeezing a mattress through a 750mm side gate. Access affects both what you can build and what you can get inside afterwards.

Don't overlook the base. A cabin needs a level, solid foundation: a concrete slab, paving slabs on a compacted sub-base, or a proprietary timber or composite system. Its footprint is usually larger than the cabin itself, so budget that ground area before ordering.

Rules, boundaries and neighbours

In England and Wales, many garden cabins fall under permitted development, but the conditions matter. Outbuildings must generally be single storey, and if any part sits within two metres of your boundary the overall height is usually capped at 2.5 metres. Otherwise, limits typically sit at 3m for most roof types and 4m for a dual-pitched roof. The building must not be forward of the principal elevation, and outbuildings together shouldn't cover more than half the land around the original house.

Scotland and Northern Ireland have their own rules, and flats, listed buildings, conservation areas and Article 4 directions all change the picture. If your cabin will include sleeping accommodation, a bathroom or a kitchen, treat it as a different project entirely — that's closer to an annexe, and planning permission and building regulations are likely to apply.

A short conversation with your local planning authority before you order will save a great deal of expense later. And it costs nothing to mention your plans to the neighbours over the fence — a cabin that blocks a view or light is the sort of thing people remember.