Self-contained annexes for family living or independent accommodation
Annexes
An annexe is self-contained living accommodation on the same plot as a main house, complete with its own kitchen and bathroom facilities, built either as a new detached structure, a conversion of an existing outbuilding, or an extension with its own entrance.
Definition
What Annexes covers
An annexe provides independent living space — commonly for an elderly relative, adult child, or as ancillary accommodation — with its own sleeping, cooking and washing facilities, distinguishing it from a garden room or home office. Because it includes cooking and sanitary facilities, an annexe is treated differently to other outbuildings for planning purposes, and typically needs its own drainage, water and electrical connections.
Suitable for
- Multi-generational households wanting independent but nearby accommodation
- Plots large enough for a new detached structure, or with an existing outbuilding suited to conversion
- Owners planning ancillary accommodation for a dependent relative
- Properties where planning policy in the area supports annexe development
Not suitable for
- Plots without room for the required separation and parking, or without planning support for an additional dwelling on site
- Situations intending the annexe to be sold or let separately from the main house without the necessary planning consent for a separate dwelling
Process
How the work is carried out
- 01
Feasibility and planning strategy
Assessing the plot, existing structures and local planning policy for ancillary accommodation, and whether a new build, conversion or extension route is best.
- 02
Design and approvals
Preparing drawings, submitting for planning permission (usually required for annexes with independent facilities) and Building Regulations.
- 03
Construction
Groundworks and structure (for a new build), or structural and fabric works to the existing building (for a conversion), followed by full services connections.
- 04
Fit-out and completion
Kitchen and bathroom fit-out, decoration, and Building Control sign-off.
Typical timescale
A new-build or conversion annexe typically takes three to six months on site depending on scale and whether it is a conversion or ground-up build, following design and approvals.
Cost guidance
Annexes typically cost £30,000–£100,000 per job in our area, with planning requirements, groundworks and the extent of new services (drainage, water, electrical supply) driving most of the range. We provide an itemised quotation once the design and planning route are confirmed.
Limitations and exclusions
- Annexes with independent cooking and washing facilities usually require planning permission, and local policy on ancillary versus independent dwellings varies between authorities
- New drainage, water and electrical connections are priced according to distance from existing supplies and are confirmed at survey
- An annexe intended for separate sale or letting from the main house is a different planning category and may not be supported by local policy
Areas we cover
Annexes across Sussex, Surrey, Kent and South London
Local property styles, planning authorities and neighbourhood coverage for annexes in each town we work in.
Related services
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FAQs
Annexes FAQs
Do I need planning permission for an annexe?
Usually yes, because independent cooking, sleeping and washing facilities mean the structure is not treated as a simple outbuilding. Some annexes intended purely as ancillary accommodation for the main house can fall under permitted development, but this depends on size, position and local policy, which we check during feasibility.
Can an existing garage or outbuilding be converted into an annexe?
Yes, provided the structure is suitable and can be brought up to full Building Regulations standard for insulation, damp-proofing and services. We assess this as part of the feasibility stage alongside a new-build option.
Will an annexe affect my council tax or mortgage?
An annexe with independent facilities may be assessed for a separate council tax band, and mortgage lenders sometimes have specific requirements for ancillary accommodation on the same title. We'd recommend checking with the local authority and your lender or conveyancer before committing to a scheme.
Discuss your annexes project
Tell us about the property and what you have in mind, and we'll arrange a site visit.
Last reviewed 2026-07-28.