Annexe Costs and Borrowing Considerations
The cost of building an annexe depends heavily on whether you are converting an existing structure or building from scratch. Converting an integral or attached double garage into a self-contained annexe typically costs £40,000 to £65,000, including partition walls, kitchen installation, bathroom fitout, insulation, electrics, and external access. Converting a detached outbuilding runs to a similar cost but may also require connection to mains services, which can add £5,000 to £15,000 if trenching is needed.
A purpose-built single-storey annexe of 35 to 45 square metres — purpose-designed to sit beside or behind the main house — typically costs £70,000 to £100,000 including groundworks, structure, and full fitout. Larger two-storey annexes or those with premium specification can reach £120,000 to £150,000. In London and the South East, costs are typically 20 to 30 per cent higher than in the Midlands or North.
On a £75,000 secured loan at 9% over 15 years, monthly repayments are approximately £761. Over 20 years at the same rate, repayments reduce to around £675. If the annexe generates rental income — a one-bedroom annexe in a suburban location might let for £700 to £1,200 per month depending on area — the rental income can substantially offset the loan repayment. Some lenders will factor potential rental income into their affordability assessment for annexe projects.
Secured loan lenders will look carefully at the CLTV position for larger annexe loans. A £90,000 secured loan on a £400,000 property with a £220,000 mortgage gives a CLTV of 77.5% — within the range of most lenders. At 85% CLTV, the maximum available would be £120,000. Building post-completion equity is also enhanced by the fact that a well-built annexe can add 10 to 20 per cent to the main property's value.
Planning Permission for Annexes
Planning permission requirements for annexes are more complex than for most other home improvements. A self-contained annexe — with its own entrance, kitchen, and bathroom — is generally considered a new unit of residential accommodation and almost always requires full planning permission, regardless of whether it is created through conversion or new build.
The planning authority will assess the impact on the neighbourhood, the design and scale of the structure, the relationship with the main house, and the potential for future subdivision and separate sale. Most authorities are amenable to annexes that are genuinely ancillary to the main dwelling and do not have the appearance of an independent property — for example, they may require that the annexe does not have its own postal address and is accessed through a shared curtilage.
A planning application for a householder annexe typically costs £258 in England and takes eight to twelve weeks for a decision. In areas with a high density of HMOs or where housing pressures are significant, local authorities may have specific policies on annexes that restrict their size or use. A planning consultant can advise on the likely outcome before you commit to detailed design.
Building regulations approval is mandatory for all annexe projects. The structure must comply with thermal, structural, fire safety, ventilation, and accessibility regulations, and a completion certificate must be issued. This certificate is essential for future sales and is required by solicitors acting for buyers. Lenders will ask for evidence of planning permission and building regulations compliance as part of the secured loan process.
