Garden rooms, explained properly before you spend anything

What counts as a garden room

The term covers anything from a flatpack timber cabin to a fully specified outbuilding with a screed base, a warm roof and its own consumer unit. Legally they are all outbuildings. Practically they behave completely differently once the temperature drops below five degrees.

The useful test is simple. Can you sit in it in January in a jumper, with the heating off for an hour, and stay comfortable? If yes, it is a garden room. If not, it is a shed with good cladding.

• Summerhouse: single skin timber, no insulation, seasonal use only
• Log cabin: thicker timber walls, better than a shed, still loses heat fast at the roof and floor
• Garden room: insulated floor, walls and roof, sealed against draughts, plastered or boarded internally
• Garden annexe: a garden room with sleeping, washing and cooking provision, which brings extra rules

How the build stacks up, ground to roof

Almost every problem people report a few years in traces back to one of four things: the base, the damp detailing, the insulation continuity or the roof covering. Get those right and the rest is finish.

Bases are the part homeowners tend to underestimate. A garden room is a heavy permanent structure and it wants a level, stable, well drained platform that will not shift with the clay under a Kent garden.

Walls, roof and the bit nobody photographs

A typical build is a treated timber frame, insulated between and often over the studs, wrapped in a breather membrane, battened to create a drainage cavity, then clad. Inside, a vapour control layer sits behind the boards so warm damp air from the room does not condense inside the wall.

Roofs are usually a warm flat roof with a single ply membrane or EPDM, or a shallow pitch hidden behind a parapet so the building keeps clean lines. Falls, upstands and outlets matter more than the covering brand. Water that cannot leave the roof will eventually find a way into the structure.

• Vapour control layer taped and continuous, not just stapled up
• Insulation carried across junctions so there is no cold bridge at floor and eaves
• Cavity behind cladding so timber can dry out
• Roof outlets sized and sited so leaves cannot block the only exit

Power, heating and internet

Most garden rooms take a dedicated armoured cable from the house consumer unit, buried at the correct depth, feeding a small consumer unit in the building. That work is notifiable under Part P, so it needs to be done and certified by a competent electrician.

For heating, a single electric panel heater or a small air conditioning unit with a heat pump function is usually plenty in a well insulated building. If you are being sold a large heat output for a small room, that is often a sign the insulation specification is thin.

Wired network back to the house beats relying on wifi through two external walls, and it costs very little while the trench is open.

Rules, in short

Most garden rooms are built under permitted development as an outbuilding that is incidental to the house, with limits on height, position and how much of the garden is covered. Sleeping accommodation, business use with visiting customers, listed buildings and conservation areas all change the picture.

Building regulations generally do not apply to a small detached outbuilding with no sleeping accommodation, but the electrical work always does, and adding a shower or a bedroom moves you into different territory. Councils interpret detail differently, so check your own local authority before you commit.

What we do and how we work

Mighty Oak Construction is based in Tunbridge Wells and builds garden rooms and outdoor transformations across Kent, Sussex and Surrey. Because the same team also does cork spray rendering, conservatory insulations, plastering and skimming and decorating, the inside of a garden room gets finished to the same standard as a room in the house rather than being lined and left.

Every project starts with a site visit and a written quote. If a plot needs a different base or the drainage says otherwise, we say so at that point rather than halfway through the build.