A small summerhouse is one of the loveliest rooms in the garden, and also one of the easiest to ruin with furniture that is slightly too big. Four walls, a couple of windows and perhaps two metres of floor space do not leave much margin for error, so the trick is to treat the whole thing like a tiny sitting room rather than an overflow store for spare garden chairs. Get the proportions right and even the smallest cabin will feel calm, welcoming and genuinely restful.

Measure first, then decide what you actually need

Before you buy a single cushion, spend ten minutes with a tape measure and a piece of chalk. Mark the door swing on the floor, note where the window sills sit and work out how much clear floor you need to walk in and sit down without shuffling sideways. In a cabin of roughly 2.4 metres by 1.8 metres, that usually leaves room for two comfortable seats and one small table, and nothing more.

Ask yourself honestly how you will use the space. A quiet reading corner needs different things from a summerhouse that hosts the occasional glass of wine with a neighbour. Be ruthless about the difference between what you want and what will physically fit. A good rule of thumb is to leave at least 60 centimetres of clear walkway between furniture, and to keep the area in front of the door completely empty.

• Note the position of any power sockets or cable entry points.
• Check the height of the window sills so seating does not block the view.
• Decide whether you need a table for drinks, for hobbies, or for both.
• Leave one wall as free as possible so the room does not feel boxed in.

Choose compact seating that earns its space

Bulky armchairs are the single biggest mistake in a small summerhouse. Instead, look for chairs with slim frames, low backs and arms that tuck neatly against a wall. Folding bistro chairs in metal or rattan are ideal because they can be hung on a hook when not in use, while a small tub chair with a narrow footprint gives you proper back support for long afternoons with a book.

If you would like to seat three or four people occasionally, consider a bench along one wall with storage underneath. It doubles as seating, keeps blankets and magazines out of sight, and visually anchors the room. A footstool is another quiet hero: it works as a perch for a visitor, a place to rest your feet, and a surface for a tray when you need one. Two well-chosen chairs will always feel more generous than three squeezed-in ones.

Tables that fold, drop or disappear

Fixed dining tables rarely suit a compact summerhouse, but there are plenty of clever alternatives. A drop-leaf table stays against the wall as a narrow console and opens out for tea and cake. A wall-mounted folding table is even better in the tightest spaces, because it takes up no floor at all once the leaf is down. For everyday use, a small round bistro table is hard to beat: it has no sharp corners, seats two comfortably and never dominates the room.

A nest of two or three occasional tables gives you flexibility without clutter. Pull one out for a cup of tea, push it back when you are finished. If you enjoy a hobby in the summerhouse, choose a table height that suits both sitting and standing, and keep a folding stool tucked beside it so you can work comfortably for longer stretches.

Soft textiles make all the difference

A summerhouse is essentially a wooden box, so it needs softening. Fabric is what turns a shed into a retreat, but choose it with damp British weather in mind. Cotton, linen and washable synthetic blends cope far better than heavy wool, which holds moisture and can smell musty after a wet winter.

• Seat pads: tie-on cushions with removable, washable covers are practical and instantly more comfortable.
• Cushions: two or three per chair in mixed sizes, in colours that pick up the garden outside.
• Throws: a light cotton or acrylic throw kept in a basket for cooler evenings.
• Rug: a flat-weave or indoor-outdoor rug in a light tone, which makes the floor feel warmer and the room feel bigger.
• Curtains or a Roman blind: useful for shading strong afternoon sun and for making the space feel finished.

Stick to a restrained palette of two or three colours. Pale walls with softer furnishings create a sense of space, while one patterned element adds interest without visual noise.

Light it gently and keep it tidy

Lighting transforms a small summerhouse after dusk. Avoid trailing cables across the floor; instead, use rechargeable LED table lamps, battery-operated lanterns or a string of warm-white fairy lights pinned neatly along the roof beams. A small lamp on a wall shelf gives a soft pool of light exactly where you need it for reading.

Finally, build in a little storage so clutter never takes hold. A wicker basket for magazines, a shallow shelf above the window for lanterns, and a lidded box for games and blankets will keep surfaces clear. Bring soft furnishings indoors during the coldest months, leave a window slightly ajar on dry days to keep air moving, and wipe down frames and cushions at the start of each season.

Furnish a small summerhouse with restraint and it will reward you every time you open the door: a calm, uncluttered corner of the garden that feels far larger than its measurements suggest.