Quick answer
A granny annexe (self-contained garden living accommodation with its own kitchen, bathroom, and independent access) typically costs £50,000-£100,000+ in the UK, substantially more than a standard garden room because it’s built to full residential building regulations standards throughout, not just partially exempt like most garden offices or gyms. Size, specification, and whether it needs planning permission (which self-contained annexes often do) are the biggest cost drivers.
What makes an annexe different from a garden room
The distinction matters because it changes the regulatory picture entirely, not just the price. A standard garden room (see our Garden Room Cost UK guide) is used incidentally to the main house — an office, gym, or studio — and often qualifies for exemptions from both planning permission and full building regulations approval. A granny annexe is different because it’s designed as self-contained living accommodation: it has its own kitchen, bathroom, sleeping space, and independent access, meaning someone could live there entirely separately from the main house. That combination is what typically removes permitted development rights and pulls the whole structure under full building regulations, much like a small residential dwelling.
Cost by specification
Basic annexe (studio-style, one room plus bathroom and kitchenette) — roughly £50,000-£70,000, covering a compact self-contained space with the essentials: a shower room, kitchenette, and open-plan living/sleeping area.
Mid-range annexe (separate bedroom, full bathroom, fitted kitchen) — roughly £70,000-£90,000, with more defined separate spaces and a fuller specification throughout.
High-spec or larger annexe (two bedrooms, higher-end finish) — £90,000-£130,000+, particularly where the design includes bespoke architectural features or a larger footprint.
These figures sit meaningfully above a standard large garden room (see our Garden Room With Bathroom Cost and Garden Room With Kitchen Cost guides for those individual additions) because an annexe combines full plumbing, a full kitchen, sleeping accommodation, and construction to full residential building regulations standards, all in one project.
Planning permission for a granny annexe
This is the area where annexes diverge most sharply from standard garden rooms. Self-contained living accommodation with independent access typically falls outside permitted development rights, meaning a full planning application is usually required — see our Garden Room Planning Permission guide for the general permitted development rules that a standard garden room can rely on, and why an annexe’s self-contained nature usually takes it outside those rules. Local authorities also vary in how they treat annexes intended for family use versus those that could function as a separate independent dwelling (which raises different questions around council tax and future sale), so it’s worth discussing your specific plans with your local planning department early.
Building regulations for a granny annexe
Because an annexe includes sleeping accommodation and full plumbing, it doesn’t qualify for the same exemptions that apply to many standard garden rooms — see our Garden Room Building Regulations guide for the general exemption thresholds (30m², no sleeping accommodation) that an annexe typically exceeds by design. In practice, this means a full building control application, inspections during construction, and sign-off before the annexe can be used, adding both time and cost compared with an exempt garden room.
Does a granny annexe affect council tax?
Potentially, yes — this is one of the most important practical differences from a standard garden room. A self-contained annexe with its own kitchen, bathroom, and independent access can be assessed for a separate council tax band, particularly if it could function as an independent dwelling regardless of who currently lives there. Some annexes qualify for a council tax discount or exemption when occupied by a dependent relative, but the rules are specific and can change, so this is worth confirming directly with your local council’s valuation office once your design is finalised, rather than assuming either way.
Annexe vs extension vs garden room: which fits your need
If the goal is a family member living semi-independently while remaining close by, an annexe is usually the right category, despite the higher cost, because it’s the only option genuinely designed for full-time independent living. If the goal is simply extra usable space (an office, gym, or guest room used occasionally), a standard garden room is very likely to be more cost-effective and faster to build — see our Garden Room vs Extension guide for how an internal house extension compares as a third option.
Frequently asked questions
How much does a granny annexe cost in the UK? Typically £50,000-£100,000+, depending on size and specification, reflecting full residential-standard construction, a kitchen, a bathroom, and (usually) planning permission costs.
Do you need planning permission for a granny annexe? Usually yes, because self-contained living accommodation with independent access typically falls outside permitted development rights — confirm with your local planning authority before finalising a design.
Does a granny annexe add council tax? It can, if the annexe could function as an independent dwelling — some annexes qualify for a discount when occupied by a dependent relative, but this should be confirmed with your local council.
What’s the difference between a garden annexe and a garden room? A garden room is typically used incidentally to the main house (office, gym, studio); an annexe is self-contained living accommodation with its own kitchen, bathroom, and independent access, which changes both the planning and building regulations picture significantly.
*Related guides: Garden Room Cost UK · Garden Room Planning Permission · Garden Room Building Regulations · Garden Room With Kitchen Cost*