A Guide to Building an Annexe
With the rising cost of housing and increasing multigenerational living, annexes have become an attractive solution for many homeowners. Whether you’re accommodating elderly parents, providing a home for an adult child, or even exploring rental income, building or converting an annexe can add both functional space and value to your property. However, navigating the legal, planning, and financial landscape is crucial to a successful project.
What Is an Annexe?
An annexe is a self-contained living space that forms part of a property, either attached to or separate from the main house. It usually includes its own sleeping area, bathroom, and sometimes a kitchenette. Annexes can be:
• Detached buildings (e.g., a converted garage or new garden building)
• Attached extensions (e.g., side or rear extensions with separate access)
• Converted internal space (e.g., a basement or loft conversion)
Planning Permission: When Is It Required?
1. Planning Permission
In general, building a new detached annexe or a significantly large extension to create an annexe requires planning permission. Your local council’s planning department will assess whether the development is appropriate in terms of:
• Impact on neighbours and the local environment
• Design and materials in keeping with the main house
• Drainage, access, and parking
• Overdevelopment of the site
2. Change of Use
If you’re converting an outbuilding or garage to residential use, it might be considered a material change of use, which also requires permission.
Important Note: Councils are often cautious about annexes becoming independent dwellings. To avoid refusal, your application should emphasize that the annexe is for ancillary use only, meaning it’s dependent on the main house (e.g., shared utilities or no separate postal address).
Permitted Development (PD) Rights
Under Permitted Development Rights, certain small-scale developments can proceed without full planning permission, provided they meet specific criteria.
Key Permitted Development Rules (for England):
For Extensions (attached annexes):
• Must not extend more than 3m (semi) or 4m (detached) to the rear (or up to 6m/8m under larger home extension scheme, subject to prior approval)
• Max height: 4m for a pitched roof or 3m for a flat roof
• Must not take up more than 50% of the garden
• Materials should be similar in appearance to the existing house
• Not allowed in listed buildings or some conservation areas
For Outbuildings (detached annexes):
• Must be single-storey
• Max height: 2.5m within 2m of a boundary, or 4m otherwise
• Cannot include sleeping accommodation under PD (technically speaking)
• Must be used for “incidental” purposes (e.g., home office, gym)
Note: Using an outbuilding as a full-time residential annexe with a bath or shower may breach PD rules, and you may need to apply for a Certificate of Lawfulness or full permission.
Converting an Existing Space into an Annexe
Garage Conversion
A common way to create an annexe. Internal work typically does not require planning but Building Regulations approval is still necessary.
Loft or Basement Conversion
These can form a studio-style annexe. Planning permission may not be required if the external structure isn’t altered significantly.
Internal Reconfiguration
Dividing part of the main home (e.g., ground floor section) into a semi-self-contained unit. Usually allowed without planning, but again, dependent on council policies.
Building Regulations
Regardless of planning permission, Building Regulations approval is always required for:
• Electrical and plumbing work
• Drainage
• Insulation and fire safety
• Accessibility
Your annexe must meet current residential standards, particularly if it will be lived in long-term.
Council Tax Considerations
If the annexe is fully self-contained, the Valuation Office Agency (VOA) may classify it as a separate dwelling, liable for its own Council Tax band.
However, you may be eligible for:
• A 50% discount if a dependent relative (over 65 or disabled) lives in it
• Exemption if it’s unoccupied or part of the main home
Costs of Building or Converting an Annexe
1. New Build Detached Annexe
• Basic timber garden annexe: £30,000–£50,000
• High-spec or brick-built: £60,000–£120,000+
• Includes foundations, plumbing, electrics, insulation, etc.
2. Garage Conversion
• £10,000–£25,000 depending on size and level of finish
3. Loft or Basement Conversion
• £25,000–£60,000 (more if complex structural work is needed)
4. Internal Conversion
• £5,000–£20,000 depending on plumbing, partitions, and layout
5. Planning & Professional Fees
• Architect/designer: £1,500–£5,000
• Planning application: £206 (England)
• Building regulations: £500–£2,000
• Legal advice (if separating title): £500–£1,500
Renting Out an Annexe
You can rent out an annexe, but this usually requires planning permission for change of use (to a separate dwelling). It also raises:
• Tax implications (income tax on rent, potential Capital Gains Tax on sale)
• Council Tax banding
• Mortgage consent (you must notify your lender)
Letting it to a family member as part of the main residence is usually less restricted.
Conclusion
Annexes can be a smart and flexible solution, whether for family accommodation or boosting property value. However, planning policy, permitted development limits, and financial costs must be carefully considered. Always consult your local planning authority and building control department early in the process to avoid costly mistakes.
If you’re unsure, hiring us with local experience can help guide you through permissions and regulations.