Garden Rooms in London: Planning, Costs & What You Can Build Without Permission

Under Class E Permitted Development (outbuildings), a garden room can be built without planning permission if all of the following conditions are met: (1) it is used for a purpose incidental to the enjoyment of the house (home office, gym, hobby room - not a separate dwelling or Airbnb let); (2) it is single-storey with a maximum eaves height of 2.5m and a maximum overall height of 4m (dual-pitched roof) or 3m (flat or other roof); (3) it is not in front of the principal elevation of the house; (4) it does not cover more than 50% of the land around the original house (including all existing outbuildings); (5) if within 2m of a boundary, the maximum height is 2.5m; (6) it is not within the curtilage of a Listed Building. In London, PD rights for outbuildings are also removed in most Conservation Areas and by many Article 4 Directions.

Planning permission is required for a garden room if: it is in a Conservation Area (most London boroughs have significant CA coverage); it would take garden coverage above 50% of the original garden area; it is within 2m of a boundary and exceeds 2.5m in height; it is to be used as a separate dwelling, rental unit, or for commercial purposes; it is attached to the house (in which case it is an extension, not an outbuilding); or if PD rights have been removed by Article 4 Direction. Planning permission for a garden room in a residential garden is generally granted if it doesn't dominate the garden, doesn't cause overlooking or loss of light to neighbours, and is of appropriate scale and materials. Refusals are most common where the proposal would cover a very high proportion of the garden or where a heritage impact is identified in a CA.

Garden rooms under 15m2 total internal floor area are generally exempt from Building Regulations if they contain no sleeping accommodation. Rooms between 15-30m2 may be exempt if they are a certain distance from the property boundary and contain no sleeping accommodation. Above 30m2, Building Regulations approval is required. In practice: if the garden room is to be habitable year-round (heated, insulated, used as a regular workspace), it is strongly advisable to build to Building Regulations standards even if technically exempt, to ensure insulation, electrics, and structural elements are correct. Any electrical installation in an outbuilding requires notification to Building Control as a Part P notifiable work.

A garden room that is adequately insulated and heated costs more to build initially but saves significant running costs and is a far more comfortable workspace in London winters. Good practice: 100-150mm insulation in walls and ceiling (PIR board or mineral wool with vapour barrier), insulated floor (50mm PIR on slab), double-glazed or triple-glazed windows and doors, and an appropriate heat source. Heating options: electric panel heaters (cheap to install, higher running costs), air source heat pump (requires outdoor unit, higher install cost, very low running costs), or infrared panels. A well-insulated 4m x 5m garden room with a small ASHP or infrared system can be heated for £300-£500 per year in London.

Garden room costs vary enormously by specification. A basic timber-framed garden room (3m x 4m, insulated, electrics, double-glazed door and windows, painted exterior, unfinished interior): £12,000-£18,000 supply and install. A mid-specification room (same size, fully lined and decorated interior, fitted lighting and sockets, heating): £18,000-£30,000. A premium specification (4m x 6m, bespoke design, full insulation to Building Regs standard, kitchen unit, WC, bifold doors, high-spec cladding): £35,000-£60,000+. London labour and access premiums add 15-25% compared to national averages. Planning application fee if required: £258. Structural engineer fee if required: £600-£1,000.