Measure the gap before you fall for the building

Almost every disappointing garden room story starts the same way: someone falls in love with a 5m × 4m cabin online, orders it, then discovers the only sensible spot in the garden is 3.6m wide. Before you browse another brochure, get outside with a proper tape measure — not your feet, not your best guess — and write down the real numbers.

Work through this properly and you will save yourself thousands:

• Boundary to boundary at the point where the building will sit, measured twice at right angles so you know the plot is truly square.
• Everything already there — sheds, trees and their root spread, drains, manhole covers, oil tanks, downpipes, the neighbour's overhanging branches.
• Access from the road or drive to the site. A narrow side return of 800mm will take a wheelbarrow but not a 4m panel; measure the tightest point, including gate posts and any step.
• Sightlines from the kitchen window, the patio and the neighbour's upstairs windows. A room that blocks the view you actually use is a room you will resent.

As a rough guide, leave at least 600mm of clear space around the outside for maintenance, guttering and air circulation, and more if you plan to walk down one side regularly.

Decide what the room is for before you decide how big it is

This is the single most common mistake. A garden room built for occasional summer reading needs far less space than one used as a full-time office five days a week. Be honest about the use, because a room that is 300mm too small feels cramped every single day.

• Home office: a desk 1.2–1.6m wide, a chair that needs roughly 900mm behind it, plus a filing cabinet or shelving. Allow 2.5m × 3m as a comfortable minimum, 3m × 4m if you take video calls or see clients.
• Studio or workshop: think about the longest piece of material you will ever bring in, and the swing of a door or a bench.
• Guest or hobby room: a single bed is 1.9m × 0.9m and you need 600mm down one side. That alone pushes you past 2.4m in one direction.
• Gym or yoga space: a mat is 1.83m long; arms outstretched add more. 3m × 3m is the realistic floor.
• Storage: measure what is going in, then add 20 per cent, because sheds always fill.

External size is not internal size

A cabin sold as "3m × 3m" is almost never 3m of usable floor inside. Walls of 45mm, 70mm or 90mm eat into the footprint on every side, and a shepherd hut's curved or tapered shape costs you more again. On a 3m × 3m log cabin with 70mm walls you lose roughly 280mm in each direction once you allow for the internal lining — around nine per cent of the floor area gone before you place a single chair.

Always ask for the internal dimensions and, if you are comparing suppliers, ask for the internal floor area in square metres. That is the number that determines whether your furniture fits. Ceiling height matters too: 2.1–2.2m is perfectly comfortable, but if you dream of a sleeping platform or mezzanine you will want 2.4m or more at the eaves.

What common sizes actually feel like

These are the sizes that come up again and again across garden rooms, log cabins and shepherd huts, and what they genuinely accommodate:

• 2.4m × 2.4m: a snug studio, a reading nook, a single-person office with a slim desk. Anything more and you will be shuffling sideways.
• 3m × 3m: the classic one-person garden office, with room for a desk, a chair, a shelf and a small sofa if you are disciplined.
• 3m × 4m: the sweet spot for most households — comfortable office plus storage, or a small guest room.
• 4m × 5m: a genuine multi-use room, a home gym, a studio with a sink and a worktop.
• Shepherd huts: typically 2.4–3m wide and around 3.6–4.8m of living space, with the tow bar adding roughly a metre on top. Lovely proportions, but narrow: plan a single run of furniture along one wall.

Access, height and the paperwork

Check how the building will reach the site. Many garden rooms and huts arrive as panels or in one piece on a lorry with a hiab; that lorry needs a clear run and somewhere to stand. A shepherd hut needs a turning circle and a level route, because it will be towed into position.

Height is worth checking early. Outbuildings in England often fall under permitted development, which generally limits eaves to 2.5m and overall height to 4m for a dual-pitch roof, with tighter limits within 2m of a boundary — but the rules differ in Scotland, Wales and Northern Ireland, and conservation areas and listed buildings change everything. If in doubt, ring your local planning authority before you order. Keep the total floor area sensible too; larger outbuildings can trigger building regulations, and anything intended as sleeping accommodation will.

The tape test — mock it out before you buy

Once you have a size in mind, mark it on the ground with canes and string, or pegs and a length of rope. Then walk inside it. Put a dining chair in the corner, stand behind it, open a pretend door. Do this on a wet afternoon as well as a sunny one, because a room that feels generous in June can feel like a box in January. If you can, leave the outline for a week and look at it every day from the house.

If the outline feels tight, it is tight — and the answer is usually to go up in one dimension rather than compromise on everything. A slightly larger building costs a little more and is used far more; a cramped one is expensive storage. Measure twice, imagine the room in use, and buy once.