Across our patch — Ashford, Maidstone, Tunbridge Wells and Folkestone & Hythe — we see the same driver again and again: ageing parents moving closer, adult children unable to get on the property ladder, or a family wanting rental income without losing the garden entirely. The right answer changes depending on which of those it is.
This page covers the planning route, the council tax and mortgage implications, and the three realistic build options — converted garage or outbuilding, a new-build garden annexe, and an internal reconfiguration of the main house — so you can work out which suits your plot and your family before committing to drawings.
Planning permission: it depends on independence
A small annexe attached to or within the curtilage of the main house, used as ancillary accommodation for a family member, can sometimes be built under permitted development rights, particularly if it is not a fully self-contained separate dwelling with its own postal address and independent access.
The moment an annexe has its own kitchen, its own front door onto the street, no shared access with the main house, and functions as a wholly independent unit, most councils will treat it as a separate dwelling requiring full planning permission, and in some cases with a planning condition tying occupation to a family member or agricultural/local need.
Council tax and the annexe discount
A self-contained annexe used by a relative can qualify for a council tax discount (commonly 50%) under rules aimed at family annexes, but this depends on occupation and the local authority's assessment, and it is not automatic — you generally have to apply and provide evidence.
If the annexe is let out commercially or sold separately from the main house, it is usually banded and taxed as its own dwelling with no discount, so the intended use should be settled before the annexe is designed, not after it is built.
Mortgage, insurance and future sale
Lenders vary in how they treat a property with an independent annexe: some are entirely comfortable, others want it noted that it will not be sold separately, and a few restrict lending on properties with multiple self-contained units. It is worth a call to your lender or broker before finalising a self-contained design, particularly if you are remortgaging to fund the build.
At resale, a well-integrated annexe is a genuine selling point for multigenerational buyers, but a poorly built or planning-uncertain one can slow a sale, so keeping a clean paper trail — permissions, building control sign-off, any conditions discharged — matters as much as the build quality.
Converted garage or outbuilding annexe
Where there is an existing garage, cart lodge or outbuilding, converting it is usually the fastest and cheapest route to extra accommodation, because the shell, foundations and often the roof structure already exist.
The work is still substantial: full insulation upgrade to current regulations, new floor slab or insulated floor, plumbing and drainage runs from scratch, and often raising eaves height or adding rooflights to get usable headroom and daylight into what was designed as storage space.
- Floor, wall and roof insulation brought up to current Building Regulations
- New drainage connection or, for detached buildings, a pumped connection back to the house
- Independent heating (often an air source heat pump or electric) sized for the annexe alone
- Fire separation and escape routes assessed if attached to the main house
New-build garden annexe
A purpose-built annexe gives you a clean-sheet layout — level access, a wet room instead of a bathtub, and a bedroom and living space sized for how the occupant actually lives, which matters a great deal for elderly relatives or anyone with reduced mobility.
It needs its own foundations, services and usually its own planning consideration under permitted development limits for outbuildings, which cap footprint, height and proximity to boundaries. On a generous Wealden plot this is rarely the constraint; on a smaller estate plot in Ashford or Maidstone it often is, and a feasibility check on siting comes before any design work.
Reconfiguring within the main house
Not every family needs a detached unit. Converting a ground-floor reception room and adjoining WC into a self-contained bedsit with an external door, or extending to create a linked but separately accessed suite, keeps daily contact easier for families providing care and avoids the planning question around a fully detached dwelling.
This route usually needs less capital outlay than a new structure and can often proceed as permitted development or a straightforward householder application, but it does mean losing a room the household currently uses, which is worth weighing against the alternative of building new.
Common questions
Can I build a granny annexe without planning permission?
Sometimes, if it stays within permitted development limits for outbuildings or extensions and remains genuinely ancillary to the main house rather than an independent dwelling. A fully self-contained unit with separate access will usually need a planning application, or at least a Lawful Development Certificate confirming its status.
Will I pay council tax on an annexe?
It depends on use. A family annexe occupied by a relative can often qualify for a discount, but a self-contained unit is generally assessed for its own council tax band, and letting it out removes any family-use discount entirely.
Can I rent the annexe out on Airbnb?
You can, but a planning condition tying the annexe to ancillary or family use would need to be lifted or was never imposed, and short-let use has its own tax and insurance implications separate from the building question.
Is a garage conversion cheaper than a new-build annexe?
Usually yes, because the shell already exists, but the saving depends on the garage's condition and whether the foundations and structure need significant work anyway — sometimes the gap narrows once that is accounted for.
Does an annexe affect the value of my house?
Generally it adds value and broadens your buyer pool to multigenerational families, provided it is built with proper consents and building control sign-off. Unauthorised or unresolved planning status is the thing that actually reduces value or causes problems at sale.