Annexe Conversion in London: Planning Rules, Costs, and What You Can Build

An annexe — a self-contained living space that is ancillary to the main dwelling — is one of the most versatile additions a London homeowner can make. Used for elderly parents, older children, live-in support, or occasional guests, a well-designed annexe adds genuine quality of life and significant value. This guide covers the planning framework, the typical scopes, and honest cost expectations.

What Counts as an Annexe?

For planning purposes, an annexe is a self-contained residential unit that is subordinate to and associated with the main dwelling. It typically includes sleeping, living, kitchen, and bathroom facilities.

The distinction between an annexe and a separate dwelling matters significantly:

• An annexe shares access with the main house (or has a separate entrance but is clearly subordinate) and is used by people associated with the main household
• A separate dwelling (for letting to unrelated tenants at market rent) constitutes a change of use and requires full planning permission as a new residential unit

Getting this distinction right is important. Councils in London have become increasingly alert to annexes being created and then let as separate HMO or rental units.

Planning Permission for Annexes

Attached annexe (part of the main house): Internal reconfigurations that create self-contained accommodation within the existing dwelling generally do not require planning permission, as there is no external change. However, if the annexe creation involves an extension, the extension's planning status applies.

Garage conversion to annexe: Garage conversion is Permitted Development in most cases (subject to conditions), but if the result is a self-contained annexe with its own entrance, the council may consider this a change of use requiring full planning permission. Always confirm with the local planning authority before proceeding.

Outbuilding converted to annexe: An outbuilding (garden room, shed, stable) converted to habitable annexe space requires full planning permission, as this constitutes a change of use from an ancillary structure to a dwelling.

New-build annexe: Always requires planning permission.

Certificate of Lawful Development: Where you believe your annexe falls within permitted development, always obtain a Certificate of Lawful Development before building. This protects you if the council later disputes the classification.

Building Regulations for Annexes

Regardless of planning status, an annexe must comply with Building Regulations:

• Part A: Structural integrity of any new or altered structure
• Part B: Fire safety (particularly the escape route from sleeping areas)
• Part F: Ventilation
• Part L: Energy efficiency (insulation, windows, heating)
• Part M: Accessibility (if applicable)
• Part P: Electrics (notification or registered electrician)
• Part H: Drainage (if new drainage connections are needed)
• Part G: Sanitation and water supply

Building Control approval is required for all annexe work involving structure, drainage, or creation of habitable space.

Council Tax

A self-contained annexe occupied by a family member who is dependent on the main household may be exempt from council tax, or subject to a 50% discount. Confirm with your local council before occupation — the rules vary and depend on who is living in the annexe and their relationship to the main household occupants.

Typical Annexe Scopes

Garage conversion to annexe Converting an integral or attached garage into a studio or one-bedroom annexe. Involves new insulation, new floor build-up, new electrics and plumbing, and typically a new external door and window arrangement.

Approximate cost (London, 2026): £25,000–£55,000 depending on specification and bathroom requirement.

Loft conversion to annexe Creating a self-contained loft suite (bedroom, bathroom, and kitchenette) with a separate entrance via an external staircase. This requires planning permission for the external stair and must comply with all fire safety requirements.

Approximate cost: £55,000–£90,000.

Ground floor rear extension as annexe Adding a rear extension specifically designed as an annexe suite — bedroom, shower room, and kitchenette. Single-storey rear extensions are often within Permitted Development for the extension itself, but the self-contained use triggers the planning discussion above.

Approximate cost: £60,000–£100,000.

Half-built or incomplete annexe completion RCB regularly receives enquiries to complete annexe conversions that a previous contractor started but did not finish. The scope depends entirely on what stage the existing work is at and whether it was done correctly.

Key Design Considerations

Soundproofing: If the annexe shares a wall or floor/ceiling with the main house, acoustic separation is important — both for practical day-to-day living and to meet Building Regulations standards.

Natural light: Annexes are often created from spaces (garages, rear additions) with limited windows. Planning the glazing to maximise natural light significantly affects how comfortable the space feels.

Separate metering: A separate electric meter and boiler for the annexe avoids disputes about utility costs when the annexe is occupied long-term.

Future flexibility: Design the annexe to be easily reconnectable to the main house if circumstances change — this protects the value of the investment.

RCB Design & Build

RCB designs and builds annexes of all types across London, from garage conversions to new-build annexe extensions. We advise on planning from the outset and manage all trades, Building Control, and party wall compliance. Contact us to discuss your annexe project.