Granny Annexe Rules UK 2026: Planning, Sizes and Costs

Last updated: September 2026 · Checked against the planning, building regulations and council tax rules published on GOV.UK, legislation.gov.uk and Planning Portal.

Do you need planning permission for a granny annexe?

Quick answer: Usually, yes. A new garden building only escapes planning permission when it is single-storey and used for a purpose "incidental" to the house, such as a gym, office or store. Somewhere a relative sleeps, cooks and washes is living accommodation, which is not an incidental use. So a new granny annexe with a bedroom, kitchen and bathroom almost always needs a planning application, and councils normally grant it on condition that it stays ancillary to the main home. Converting an existing garage or outbuilding is often the exception.

The granny annexe is one of the most misunderstood builds in North London. The internet is thick with claims that you can put a fully fitted annexe in the garden under permitted development, no questions asked, and companies selling garden buildings have every incentive to let you believe it. Getting it wrong can mean an enforcement notice against a building your parent is already living in. This guide sets out the rules for England as they stand in September 2026: when an annexe needs planning permission, the difference between incidental and ancillary use, what "self-contained" means, lawful development certificates, building regulations, council tax, and what you can do with the annexe later.

Want one built, not just explained? This guide covers the rules. For what we build, what it costs and how the process runs, see annexe design and build in London.

When does a granny annexe need planning permission?

It depends on how the annexe is created. There are six realistic routes, and their planning positions differ sharply.

Route	Planning position
New detached annexe in the garden	Planning application almost always required. Living accommodation is outside the permitted development rules for garden buildings
Converting an attached or integral garage	Usually no application if the work is internal, the building is not enlarged and the annexe stays part of the household. Check for a planning condition keeping the garage for parking
Converting an existing detached outbuilding	Often lawful without an application while the use stays ancillary to the house. A lawful development certificate proves it
Annexe as an extension to the house	Householder application, or sometimes permitted development as an extension, because the rooms connect to the main home
Two-storey detached annexe	Always needs planning permission. Permitted development outbuildings must be single storey
Mobile or caravan-type annexe	Can be lawful without an application if the unit meets the legal definition of a caravan and is used as part of the household. Get a certificate before you buy

Some homes have fewer shortcuts. Flats and maisonettes have no householder permitted development rights. On newer estates the original planning permission often carries conditions that keep a garage for parking or remove permitted development rights, and a listed building needs listed building consent for the works. Check the planning history of your own address before relying on any route that avoids an application.

Incidental or ancillary: what is the difference?

These two words decide most annexe cases, and they are not interchangeable.

Incidental is the test for permitted development. Class E of the permitted development order allows a garden building required for a purpose "incidental to the enjoyment of the dwellinghouse": a gym, a home office, a studio, a store, a garage. The government's technical guidance for householders says this does not cover normal residential uses. It names separate self-contained accommodation, and the use of an outbuilding as primary living accommodation such as a bedroom, bathroom or kitchen. Incidental uses are things you do because you live in the house. Living itself is not one of them.

Ancillary describes the annexe. Ancillary accommodation is living space that works as part of the main household even if it has its own front door: a parent, grandparent or adult child lives there, shares the garden and the drive, and the two buildings are occupied as one home. Because of that relationship an annexe is not a new dwelling in planning terms, which is why councils approve annexes in back gardens where they would refuse a new house.

Put the two together and the position is clear. A new building for ancillary living accommodation is not incidental, so it cannot use permitted development and needs a planning application. An existing building within the grounds of the house can usually be used as ancillary accommodation without an application, because a family member living in part of your home is not a change of use. That is why the conversion routes in the table are easier than the new-build one.

One warning on the conversion route. An outbuilding put up under permitted development and turned into living accommodation soon afterwards is not protected. The building was never really required for an incidental purpose, and the council can treat it as an annexe built without permission.

Can I build a granny annexe under permitted development?

Not a new one that someone will live in. Planning Portal states that the outbuilding rules do not allow use as separate self-contained living accommodation, and the technical guidance rules out bedrooms, kitchens and bathrooms as primary living space. So a new-build granny annexe cannot rely on garden-building permitted development, however neatly it fits the size limits. If a salesperson tells you otherwise, ask for it in writing.

For reference, the physical limits that apply to genuine garden buildings are: single storey only; eaves no higher than 2.5m; overall height no more than 4m with a dual-pitched roof or 3m with any other roof, dropping to 2.5m within 2m of a boundary; nothing forward of the principal elevation; no verandas, balconies or raised platforms; and no more than half the land around the original house covered by outbuildings and extensions combined. In a conservation area an outbuilding at the side of the house needs permission, and within the grounds of a listed building every outbuilding does. Our permitted development rules 2026 guide covers the full picture.

What makes an annexe self-contained, and why does it matter?

An annexe is self-contained when someone can live in it without using the main house: it has its own sleeping space, kitchen, bathroom and entrance. A granny annexe is normally self-contained in that physical sense, because the point is that a parent can cook their own breakfast and bathe in private with family close by. A kitchen does not by itself turn an annexe into a separate house. Planning looks at how the two buildings are used together, not at the list of appliances.

Being self-contained matters in three ways:

• Planning. Self-contained living accommodation is outside the permitted development rules for garden buildings, so a new one needs an application. The council then looks at how the annexe relates to the house: shared access, shared garden and parking, no separate plot, and an occupier who is part of the household. Expect a condition tying its use to the main home.
• Council tax. A self-contained unit is given its own council tax band, although a discount or exemption usually applies to a family annexe. The rules are set out below.
• Later use. An annexe that is let to an unconnected tenant, fenced off with its own access or sold apart from the house has become a separate dwelling. That is a change of use needing its own planning permission, and it breaches the ancillary condition.

To stay clearly on the right side of the line, keep one planning unit: one shared garden, one access from the road, no boundary fence between the two buildings, and an occupier who belongs to the household.

Do I need planning permission for a garden annexe for a relative?

For a new building, yes. Who will live there does not change the permitted development position: a garden annexe for a parent is still living accommodation. What the family link does is make the application far easier to justify, because it shows the annexe will be used as part of the household and not as a separate home.

A good application explains who will live in the annexe and how they are connected to the household, shows the shared access and garden on the plans, keeps the building clearly smaller than the house, and deals with the neighbours' privacy and light. It is normally made as a householder application to your borough, with a target of eight weeks for a decision once the application is validated. Approvals usually carry a condition that the annexe is only occupied as accommodation ancillary to the main house and is not sold or let separately. That condition is not a technicality. It is the legal basis on which the annexe exists.

Do I need a lawful development certificate for an annexe?

A lawful development certificate is the council's written confirmation that something is lawful without planning permission. It is optional, but for an annexe it is the document that ends the argument. It is worth having in three cases: converting a garage or existing outbuilding into ancillary accommodation, siting a caravan-type annexe, and proving that an annexe which has been in use for years is lawful.

The council decides a certificate on the facts and the law, not on planning merits, so the design is not up for debate. A certificate for proposed householder works costs half the planning fee, £274 from 1 April 2026, and a certificate for an existing use or building costs the same as the full application. When you sell, it is the paper the buyer's solicitor asks for. Our lawful development certificate guide explains the process.

Does a granny annexe need building regulations approval?

Yes, whatever the planning route. Small detached garden buildings can be exempt from building regulations, but only where they contain no sleeping accommodation. An annexe does, so it is checked for structure, fire safety, insulation, ventilation, drainage and electrics as habitable accommodation. A garage or outbuilding conversion needs approval too, because it is becoming a place to live. Building control inspects the work in stages and issues a completion certificate at the end. Without that certificate the annexe becomes a problem the day you sell.

How is a granny annexe treated for council tax?

A self-contained annexe is normally given its own council tax band by the Valuation Office Agency, so in principle it gets a separate bill. For a family annexe in England, two national rules usually reduce or remove that bill:

• The 50% annexe discount. In force since 1 April 2014. The bill for the annexe is halved if the people in the main house use the annexe as part of their own home, or if the person living in the annexe is a relative of the person who pays council tax on the main house. Relative is defined widely and includes partners, parents, children, grandparents, grandchildren, brothers, sisters, aunts, uncles, nephews and nieces.
• The dependent relative exemption (Class W). The annexe pays no council tax at all if it is the main or only home of a dependent relative of someone living in the main house. A dependent relative is one who is aged 65 or over, severely mentally impaired, or substantially and permanently disabled.

Since a granny annexe usually houses a parent aged 65 or over, the full exemption is the one most families should check first. An empty annexe can also be exempt (Class T) where planning rules prevent it being let separately from the main house. None of this is automatic. Apply to your borough's council tax team once the annexe is occupied, and tell them if the occupier changes, because the relief depends on who lives there and not on the building. The government's plain English guide to council tax on GOV.UK sets out both the discount and the exemption.

Can I rent out or sell a granny annexe later?

Check the decision notice first. Most annexe permissions carry a condition that the building is only occupied as accommodation ancillary to the main house and is not let, sold or occupied as a separate dwelling. The condition runs with the property, so it binds you and anyone who buys from you.

• Selling the house and annexe together is fine. The buyer takes on the same condition, and their solicitor will ask for the planning permission, the building regulations completion certificate and any lawful development certificate.
• Selling the annexe on its own creates a separate dwelling. That needs planning permission in its own right, which a back-garden plot rarely gets.
• Letting it to an unconnected tenant as their independent home ends the ancillary use and breaches the condition. A family member living there as part of the household does not.
• Levy clawback. If you claimed the residential annexe exemption from the Community Infrastructure Levy, it is withdrawn if, within three years of completion, the annexe is let, the house or the annexe is sold separately from the other, or the main house stops being used as a single dwelling. The levy then becomes payable.

You can apply to the council to vary or remove a condition. Asking to lift an ancillary-use condition is in effect asking for a new house in the garden, and it is judged that way.

Can I build a two-storey granny annexe?

A two-storey detached annexe always needs planning permission, because the single-storey rule for permitted development garden buildings is absolute. Councils approach them warily: a two-storey building with a kitchen and bathrooms in a back garden looks very like a new house, and first-floor windows overlooking neighbouring gardens are a common objection.

Where two storeys of accommodation are needed, the more approvable route is usually a two-storey extension to the main house with a connecting door. It reads as one enlarged home and keeps everything under one roof for building regulations, insurance and resale. The site and the street usually decide which will pass.

What does a granny annexe cost?

This guide does not quote build prices. They depend on the route, the size and the site, and we keep them in one place so they stay current: see granny annexe costs and builders in London. What can be stated here are the charges that apply whoever builds it:

• Planning fee. The national fee schedule in force from 1 April 2026 sets £548 for a householder application to enlarge or alter a house, and £272 for building within the garden for purposes ancillary to the house. The council confirms which applies to a detached annexe when it validates the application.
• Lawful development certificate. £274 for proposed householder works.
• Building control. A charge for checking the plans and inspecting the work, set by your council's building control team or the registered building control approver you appoint.
• Community Infrastructure Levy. A residential annexe built within the grounds of your main home is exempt, but only if you claim the exemption and submit a commencement notice before work starts.
• Council tax. Often nil or halved for a family annexe, as set out above.

On the build itself, four things move the price most: the route (a conversion reuses a shell, a new building does not), the length of the drainage and service runs from the house, access to the back garden for materials and plant, and the specification of the kitchen and bathroom. Every statutory fee is listed on our planning fees page. For the neighbouring routes, see the garage conversion cost guide and the garden room cost guide.

Should I build a garden room instead of an annexe?

If nobody will sleep there, yes, and it is a far simpler project. A garden room used as an office, gym or hobby space is what garden-building permitted development exists for: usually no planning application, no ancillary-use condition and no separate council tax band. Plenty of families start there and add plumbing ambitions later, which is the expensive way round, because a building never designed for drainage is hard to retrofit with a bathroom and kitchen. Decide the real requirement first. Our garden rooms page covers that route, and our guide to garden room planning permission explains where the permitted development line sits.

Frequently asked questions

Do I need planning permission to convert my garage into an annexe?

Often not. Planning Portal's position is that converting a garage does not usually need planning permission where the work is internal and the building is not enlarged, and ancillary accommodation for a family member keeps the garage part of the same home. Check first, though: some estates carry planning conditions requiring garages to remain as parking, and turning a garage into a separate house needs permission whoever lives in it. A lawful development certificate (£274) settles it in writing.

Is there a maximum size for a granny annexe?

There is no fixed national maximum for an annexe that goes through a planning application. The council judges whether it is subordinate to the main house in size and clearly dependent on it, and an annexe that approaches the size of a small house invites the question of whether it is really a separate dwelling. The fixed numbers belong to permitted development garden buildings: single storey, 2.5m eaves, 4m or 3m overall height, 2.5m within 2m of a boundary, and no more than half the land around the original house covered.

Does an annexe add value to my house?

It can, because it widens the pool of buyers to families with elderly parents or adult children at home. The condition is paperwork. An annexe with its planning permission or lawful development certificate and a building regulations completion certificate is an asset. One without them can stall a sale while the buyer's solicitor asks questions nobody can answer.

Are wooden or prefab granny annexes treated differently by planners?

No. Planning tests size, siting and use, not construction method. A timber or prefab annexe delivered in sections follows the same rules as a brick one: as a garden building it must stay within the permitted development limits and an incidental use, and as living accommodation it needs a planning application whatever it is made of. Where prefab construction helps is speed on site, not an easier planning route.

Do I need planning permission for a granny flat in my garden?

If "granny flat" means somewhere a relative will live, sleeping, cooking and washing, then yes in almost all cases. The garden-building rules that let you put up a gym or office without an application exclude self-contained living accommodation. An application for a relative's annexe stands the best chance when the annexe stays clearly ancillary to the main house: shared garden, no separate boundary fence and no separate access.

Can I put a mobile home or caravan-style annexe in my garden?

This is the "caravan loophole", and it is real but narrower than the marketing suggests. A twin-unit structure counts as a caravan in law if it is made of no more than two sections assembled on site, can be moved by road when assembled, and is no more than 20m long, 6.8m wide and 3.05m high inside. Stationing one in the garden for use as part of the household can be lawful without a planning application, because it is a use of the garden and not a building operation. The limits matter: if it is let separately, occupied by a separate household or stops being ancillary to the main house, it needs permission like anything else. If you are relying on this route, get a lawful development certificate confirming the position before you spend money.

What happens if an annexe is built without planning permission?

The council can take enforcement action, and the time limit for doing so is now long. For building work substantially completed on or after 25 April 2024, and for changes of use to a dwelling made from that date, the council has ten years to enforce. The old four-year rule only survives for breaches before that date. Waiting it out is not a plan. Our guide to building without planning permission covers retrospective applications and what happens when you sell.

Do I need drawings for an annexe application?

Yes: a location plan, block plan, floor plans and elevations at minimum. For an annexe we also recommend a supporting statement on the ancillary-use point, because that is what the council needs to be satisfied about. Our drawings and plans service produces the full application pack.

An annexe is a planning argument, a construction project and a piece of family planning all at once, and the order you tackle them in decides how smoothly it goes. The Extension Company is the extensions studio of Pine Grove Design & Build Ltd. We design annexes, make the planning application and build them to a fixed price, and North London projects are built by Pine Grove's own team. See granny annexe builders in London for what we build and what it costs, or book a free site visit and we will look at your garden and your property's planning history before you commit to anything. 020 3051 9430.