Annexe Conversions in London: Creating Self-Contained Living Space

A residential annexe is a self-contained unit of accommodation that: has its own access (typically from outside the main house, or via a shared hallway); has its own kitchen or kitchenette; has its own bathroom and WC; and has its own habitable living space (bedroom, sitting area). The key distinction between an annexe and a spare bedroom is the self-contained nature of the unit — a bedroom, even with an en-suite, is not a self-contained annexe if it does not have its own kitchen. The planning and legal status of an annexe matters: a true self-contained annexe in London may require planning permission (as it represents a change of use — from residential to two separate residential units); and may trigger council tax liability as a separate dwelling. An annexe that is physically connected to the main house and is genuinely ancillary to the main dwelling (used by a family member, with shared access to the main house) is treated differently under planning policy than a fully independent self-contained unit.

The main annexe types available in London properties: Garage conversion annexe: converting an integral garage (under the main house or attached to it) into a studio annexe with kitchenette, bathroom, and living/sleeping space. The most cost-effective annexe creation route in properties with a garage. Costs: £35,000-£65,000 for a full studio annexe. Outbuilding annexe: converting an existing garden building or creating a new garden studio with annexe facilities. Requires garden space, planning permission in most cases, and separate utility connections. Basement annexe: converting an existing basement or excavating a new basement to create an annexe — the most expensive option but creates significant additional accommodation below ground. Costs: £80,000-£200,000+ depending on whether excavation is required. Extension annexe: creating a new ground-floor extension specifically as an annexe — accessed from outside but adjacent to the main house. Often used where the existing structure has no suitable existing space. Loft annexe: less common, as lofts are usually connected to the main house circulation — but a loft conversion with a separate staircase from outside is occasionally used. Internal annexe (within the main house): dividing a large house into main house and annexe using an internal door — technically easier but planning policy in some London boroughs treats this as the creation of a new dwelling, requiring planning permission for change of use.

Whether a London annexe requires planning permission depends on its planning classification. The critical planning distinction is between: A dwelling house annexe (C3 use, ancillary to the main house): planning policy across London broadly accepts annexes that are physically and functionally ancillary to the main dwelling — used by a dependent relative or adult family member, accessed primarily through the main house, and physically connected. Most London councils accept that an annexe of this type is C3 use (same as the main house) and may fall within Permitted Development if within the constraints on extensions and outbuildings. A separate dwelling (also C3 but as a self-contained independent unit): if the annexe is truly self-contained with its own locked front door and is intended for independent occupation, planning permission for a new dwelling is required. The distinction affects: council tax (a separate dwelling is rated separately); planning policy (a new dwelling must meet housing density, design, and space standards); and Stamp Duty Land Tax on sale. In practice, the planning requirement for a London annexe depends on: whether it has its own external access; whether it is physically separated from the main house; the borough's specific planning policies on ancillary accommodation; and whether the Article 4 direction applies. Always check with the local planning authority or a planning consultant before proceeding.

Garage conversion to studio annexe: £35,000-£65,000 (includes: converting the garage space, adding kitchenette, bathroom, bedroom/living space, new external access, all services). Garden outbuilding annexe (new build): £60,000-£120,000 (includes: the new structure, all services connections, foundations, insulation, bathroom and kitchen, planning and building regulations fees). Extension annexe (new single-storey extension designed as a self-contained unit): £80,000-£130,000. Basement annexe (existing basement conversion, not excavation): £60,000-£100,000. New basement with excavation: £150,000-£300,000+. The biggest cost variables are: whether a new structure must be built or an existing space converted; the extent of services connections required (distance from existing gas, water, electricity, and drainage); the level of finish specified; and whether planning permission is required (add £5,000-£20,000 for planning application, consultant fees, and any required heritage or other specialist reports).

A well-designed annexe in London generates rental income that can significantly offset the project cost. Typical rental values (2025 London market): Studio annexe (30-40m2, self-contained): £1,200-£1,800/month in inner London; £900-£1,400/month in outer London. 1-bedroom annexe (40-60m2): £1,500-£2,200/month in inner London; £1,100-£1,700/month in outer London. Rental yield calculation for a £65,000 garage conversion annexe yielding £1,400/month: annual rental income £16,800; yield on cost: 25.8%. Even after costs (letting agent fees, insurance, maintenance), the net yield is exceptional compared to other investment classes. Tax on rental income: the rental income from an annexe in your own home is taxable income. However, if you rent the annexe under the government's Rent a Room Scheme (letting a furnished room or annexe in your main home), you can receive up to £7,500/year tax-free. Beyond £7,500, the income is taxed as rental income in the normal way. Council tax: a self-contained annexe used by a family member may be eligible for a council tax exemption or 50% discount — check with your London borough.