Do I need building regulations to convert a garage to an annexe?

Yes — a garage-to-annexe conversion requires full building regulations approval. The conversion involves: Part A (structure — floor and walls); Part B (fire safety); Part C (moisture resistance — DPM); Part F (ventilation); Part G (sanitation — bathroom and kitchen); Part H (drainage); Part L (energy efficiency — walls, floor, roof insulation); Part P (electrics). You must either submit Full Plans or a Building Notice to LABC or an Approved Inspector before starting work.

Can I convert a semi-detached garage into an annexe?

Yes, but a semi-detached (attached to the main house) garage follows different planning rules to a detached garage. Conversion to ancillary living accommodation (bedroom or living room for the main house) may be permitted development under Class A. Conversion to a self-contained annexe with independent kitchen and bathroom requires full planning permission. The structural party wall between the garage and main house must be assessed, and Party Wall Act notices may be required if the conversion affects the shared structure.

Is it cheaper to convert a garage or build a new garden annexe?

Garage-to-annexe conversion is typically £5,000–£20,000 cheaper than a new-build garden annexe of equivalent size, because the shell, foundations and roof already exist. However, the cost advantage narrows if the existing slab and walls need significant upgrading. A new-build annexe on a clean site with a modern SIP or timber frame often outperforms a retrofitted garage on thermal performance and internal quality for a similar overall cost.

Can I put a garage annexe on Airbnb in London?

No — a garage annexe with planning permission will typically have a condition requiring occupancy ancillary to the main house. Short-term commercial letting (Airbnb) breaches this condition. London boroughs have increasingly active enforcement on unauthorised short-term lets. Additionally, properties let on Airbnb for more than 90 nights per calendar year in Greater London require a separate planning permission (Deregulation Act 2015 exemption applies for the main dwelling only, not annexes).

Garage to Annexe Conversion Cost London 2025 | Builderr

How Much Does It Cost to Convert a Garage to an Annexe in London?

Converting a garage into a self-contained annexe in London costs £35,000–£80,000 depending on size, existing structure and finish. A basic 20m² garage annexe with bedroom, bathroom and kitchenette costs £35,000–£55,000. A 30m² premium annexe with full kitchen and high-end fit-out costs £60,000–£80,000. Planning permission is almost always required.

Garage-to-annexe cost breakdown

The key cost items for converting a garage into a self-contained annexe in London: structural assessment and any underpinning or strengthening (£1,500–£5,000); new foundations or floor slab if the existing slab is inadequate (£3,000–£8,000); insulation to floor, walls and roof (£3,000–£7,000); partition walls and ceiling (£2,000–£5,000); replacement garage door with wall and window (£3,000–£8,000); bathroom fit-out (£5,000–£12,000); kitchenette or full kitchen (£3,000–£15,000); mains services (electrics, plumbing, heating) (£5,000–£15,000); internal finishes (plasterboard, skim, paint, floor) (£3,000–£8,000); external works (path, lighting, landscaping) (£1,000–£5,000). Total: £30,000–£75,000 depending on specification.

Planning permission for garage-to-annexe conversion

Converting a garage into a self-contained annexe (with independent sleeping, bathroom and kitchen) requires full planning permission in London — it is a change of use to a new dwelling. The application is assessed on: residential amenity (parking loss, overlooking); design (does the annexe elevation match the street character?); housing policies (is backland development appropriate here?). Most outer London boroughs (Barnet, Bromley, Croydon, Havering) consider garage-to-annexe conversions favourably where: a car-free condition is accepted; the garage is not a character garage in a conservation area; the design respects neighbouring amenity; the annexe is genuinely for dependent family use. Inner London boroughs are more restrictive. Note: a basic garage-to-rooms conversion (adding a bedroom or living room for the main house, no independent kitchen or bathroom) may be permitted development under Class A.

Structural challenges in garage-to-annexe conversion

Garages present several structural challenges not present in new-build annexe construction. Existing garage floors are typically 100mm concrete slabs without insulation or a damp-proof membrane — these must be upgraded (break out and re-pour, or overlay with insulated screed). Garage walls are often single-skin blockwork with no insulation cavity — internal insulated dry-lining (50mm PIR + plasterboard) is the standard retrofit. The garage roof, if flat, needs re-waterproofing and additional insulation. The existing garage door opening must be infilled with a block cavity wall (or structural frame if spanning more than 3m). A steel beam may be required to span the door opening structurally. Access from the main house and separate external access must both be considered in the design.

Adding value vs. losing a garage

Losing a garage reduces property value in London — typically by £5,000–£20,000 depending on location. A well-specified self-contained annexe in outer London (Barnet, Bromley, Croydon) typically adds £40,000–£70,000. Net value impact is therefore positive in most cases. However, in inner London where parking is limited and no off-street parking is available, losing a garage can be more damaging. The planning condition on a garage-to-annexe permission sometimes requires a car-free undertaking — accepting that the occupant of the annexe will not seek a parking permit. This is worth considering in areas where residents' parking is contested.