A spare room at the bottom of the garden

A log cabin tucked at the end of the lawn is one of the most cost-effective ways to add a genuine spare bedroom, and it comes with a perk no loft conversion can match: your guests get their own front door, their own bathroom run and the freedom to stay up late without disturbing the house. The catch is that a cabin built for summer evenings will not automatically work in January. Treat it as an annexe rather than a shed, and it will be comfortable in every month of the year.

Insulate the walls, floor and roof — in that order

Most garden cabins use interlocking timber between 28mm and 44mm thick. That keeps the rain out but holds very little warmth, so additional insulation is the single biggest improvement you can make. Plan the work from the walls outwards and don't skip the floor, which is where cold really bites.

• Walls: build a stud frame inside the cabin, fill it with 50–75mm rigid foam boards or mineral wool, then finish with plasterboard or tongue-and-groove cladding. Leave a small service void for wiring.
• Floor: lay rigid boards between battens, or use an insulated subfloor panel system, then finish with laminate, engineered oak or vinyl. Add a rug for warmth underfoot.
• Roof: fit 100mm of insulation between the joists and keep a ventilated gap above it so the structure can breathe. Heat rises, so this is money well spent.
• Windows and doors: if you are specifying a new cabin, choose double-glazed units and a well-fitting door with a decent threshold seal. Draughts around a single-glazed door will undo much of your other work.

Also seal gaps where the wall panels meet the floor and ceiling. A tube of frame sealant and a few hours' work will noticeably reduce heat loss.

Heating that copes with a February frost

A guest room needs heat that comes on reliably, not a heater that has to be coaxed. The simplest solution is an electric panel heater or oil-filled radiator on a thermostatic control, set to a background temperature of around 15°C and raised to 20–21°C when someone is staying. A wall-mounted convector with a timer means the room is warm before your guests arrive.

If you want a focal point, a small wood-burning stove transforms the space on a winter evening, but it must be installed with proper clearances, a non-combustible hearth and a carbon monoxide alarm. Never treat a stove as the primary heat source for a room someone sleeps in; keep a secondary electric heater as backup. Fit a smoke alarm and a CO alarm regardless of what you install, and test both before every stay.

Ventilation and damp: the quiet essential

Insulated cabins that are sealed tight will trap moisture from breathing, drying towels and kettle steam. Left unchecked, that shows up as condensation on the glazing and a musty smell in the mattress. Trickle vents in the windows, an extractor fan if you have a shower or kitchenette, and a small dehumidifier running on a timer will keep everything fresh. Open a window for ten minutes each morning during a stay — brief, full air changes are more effective than leaving one slightly ajar all day.

If the cabin will sit empty for weeks at a time, leave the heating on a frost setting of about 5°C and keep a dehumidifier on low. It costs very little and prevents cracked timber and ruined bedding.

Furnish simply so it feels like a room, not a shed

Keep the layout calm and uncluttered. A standard double bed (4ft 6in) is usually the right choice, though a zip-and-link king gives you flexibility if the room doubles as a study. Choose a bed with a slatted base — a mattress sitting directly on a solid platform will sweat and go damp.

• Storage: an ottoman bed or a pair of under-bed drawers swallows suitcases, spare bedding and towels without eating floor space.
• Lighting: warm white bulbs (around 2700K), two bedside lamps, a dimmer on the main light and a low-level night light for the walk to the door.
• Soft furnishings: thermal-lined curtains across the windows and a spare throw folded on the bed. Both make a cold evening feel cared for.
• A chair and a small table: somewhere to put a cup of tea and a book is what turns a bedroom into a retreat.

Practical touches guests actually notice

Run power properly: a dedicated armoured cable from the house, its own consumer unit with RCD protection and enough sockets, including a couple with USB ports. A small kettle, a couple of mugs and a bottle of water on arrival cost almost nothing and make people feel welcome. Finally, light the path from the back door to the cabin — solar stake lights are fine in summer, but a proper mains-fed or motion-sensor light is kinder in the darker months. Do those things and your garden building stops being a fair-weather novelty and becomes the guest room everyone asks for.