Annex (annexe)

An annex, usually spelt annexe in British English, is a secondary living space attached to or built within the grounds of a main house, with some or all of the facilities needed to live independently. For council tax, the Council Tax (Reductions for Annexes) (England) Regulations 2013 treat an annex as a separate dwelling that forms part of a single property with the main home.

In property, the word covers granny annexes, converted garages, garden annexes and self-contained flats within a house. Outside property, an annex is also an addition to a document (such as "Annex A" of a report), and "to annex" is a verb meaning to take control of territory. This entry deals with residential annexes in England.

Annex or annexe: which spelling is correct?

Both are correct. British English traditionally uses "annexe" for the noun (a building or living space) and "annex" for the verb, while American English uses "annex" for both. UK estate agents, councils and HMRC use the two spellings interchangeably, so a listing for a "house with annex" and one for a "house with annexe" describe the same thing. Legislation, including the 2013 council tax regulations above, uses "annex".

What counts as an annex in a house?

An annex is any part of a property that can function as a separate home while sitting on the same plot as the main house. Common forms include:

• Attached annex: an extension or wing with its own living space, connected to the main house by an internal door or a shared hallway.

• Converted annex: a garage, loft or outbuilding converted into living accommodation.

• Detached garden annex: a standalone building in the garden, sometimes a modular or mobile unit.

A granny flat is the informal name for an annex occupied by an older relative. It has no separate legal meaning: the same planning, council tax and tenancy rules apply.

When is an annex a separate dwelling?

Whether an annex counts as a separate dwelling decides how it is taxed and whether it can be let. The Valuation Office Agency (VOA) treats an annex as a self-contained unit when it has been built or adapted for use as separate living accommodation. In practice the VOA looks for space to live and sleep, somewhere to prepare food, washing facilities and a WC. A shared hallway, shared utilities or the lack of a separate front door does not stop an annex being self-contained, but an annex reached only through the main house's living room usually is not.

Who lives there is irrelevant to this test. An annex used by the homeowner's teenage children can still be self-contained if it has the facilities. Removing a cooker while leaving the plumbing in place rarely changes the VOA's view; the alteration has to be substantial and difficult to reverse.

Do you need planning permission for an annex?

Usually, yes. Permitted development rights for outbuildings cover buildings used for purposes incidental to the house, and the government's householder technical guidancestates that this excludes primary living accommodation such as a bedroom, bathroom or kitchen. A garden building fitted out as a self-contained annex therefore needs planning permission, or a Lawful Development Certificate where the owner can show the proposal is lawful without one.

Councils normally approve annexes as ancillary accommodation: space used by the household in the main house, not as an independent home. Many permissions carry a planning condition tying occupation to the main dwelling. Using a single house as two or more separate dwellings is a material change of use under section 55(3)(a) of the Town and Country Planning Act 1990, so letting an ancillary annex to an unrelated tenant generally needs a fresh planning permission or the removal of the condition.

Building regulations

Any annex intended for people to sleep in must meet building regulations covering structure, fire safety, drainage, ventilation and energy efficiency. The exemption for small detached outbuildings under 30 square metres does not apply once a building contains sleeping accommodation. Building regulations approval comes from the council's building control team or a registered building control approver, and a completion certificate is the document buyers, lenders and insurers ask to see.

Council tax on an annex

A self-contained annex is normally banded separately from the main house, which means two council tax bands on one plot. Three national rules then reduce or remove the annex bill in England.

50% reduction for family use: under the 2013 annex regulations, the annex bill is halved when the annex is the sole or main residence of a relative of the person liable for council tax on the main house. The same 50% reduction applies when the household in the main house uses the annex as part of its own home rather than letting it.

Class W exemption for a dependent relative: under article 3 of the Council Tax (Exempt Dwellings) Order 1992, an annex is fully exempt when it is the sole or main residence of a dependent relative of someone living in the main house. The Order defines a dependent relative as one aged 65 or over, severely mentally impaired, or substantially and permanently disabled.

Class T exemption for an empty annex: an unoccupied annex is exempt where it cannot be let separately from the main house without a breach of planning control.

None of these applies when the annex is let to someone outside the family. A tenant who occupies a separately banded annex as their main home is liable for its council tax at the full rate, less any personal discount they qualify for, such as the single person discount.

Can you rent out an annex?

You can, provided the planning position allows it. Check the original permission for an ancillary use condition before advertising. From working with self-managing landlords across the UK, we find this is the step most often missed: owners who inherit or buy a house with an annex assume that a kitchen and front door mean it can be let, and only discover the condition when a neighbour complains or a buyer's solicitor raises it.

Which tenancy applies?

The tenancy type depends on how the annex relates to the owner's own home, not on what the agreement is called.

Detached annex, owner living in the main house: the resident landlord exception in paragraph 10 of Schedule 1 to the Housing Act 1988 only covers lettings in the same building as the landlord's home. A tenant in a separate building in the garden therefore holds an assured tenancy, which since 1 May 2026 is an assured periodic tenancy with no fixed term and no Section 21 route to possession.

Attached annex, owner living in the main house: where the annex is part of the same building, the resident landlord exception usually applies, so the letting is not an assured tenancy. If the occupier also shares living space such as a kitchen with the owner, the arrangement is closer to having a lodger.

Owner not living on site: the annex is an ordinary buy-to-let dwelling and the full Renters' Rights Act 2025 regime applies, including the ban on rental bidding and the restrictions on rent increases.

Rent a Room relief and annexes

The Rent a Room scheme lets resident owners earn up to £7,500 a year tax-free from furnished accommodation in their home, but HMRC's guidance excludes homes converted into separate flats. Rental income from a self-contained annex is normally taxed as property income, and landlords using August consistently tell us this is the point where an informal family arrangement turns into a business they need to record and report. Our guide to taking in a lodger in 2026 covers the arrangements that do qualify.

Compliance for a let annex

A self-contained annex let as a separate dwelling needs its own Energy Performance Certificate rated E or above, a gas safety certificate if it has gas appliances, an electrical installation condition report, and working smoke and carbon monoxide alarms. The deposit must be protected in a government-approved scheme within 30 days. The government has confirmed that privately rented homes must reach EPC band C by 1 October 2030, so an annex with poor insulation is worth assessing now.

Stamp duty and capital gains tax on a house with an annex

Stamp duty: Multiple Dwellings Relief, which once reduced stamp duty on a house bought with an annex, was abolished for purchases completing on or after 1 June 2024, as confirmed on GOV.UK. A self-contained annex can still affect the higher rates for additional dwellings. Under HMRC's subsidiary dwelling rule, an annex is ignored for the higher rates when the main house accounts for at least two-thirds of the price, so a buyer replacing their main home does not pay the surcharge simply because the house comes with an annex. The August stamp duty calculator shows the rates that apply to an additional property.

Capital gains tax: private residence relief covers an annex used as part of the owner's home. Letting the annex can make part of the gain taxable when the property is sold. Lettings relief of up to £40,000 remains available only where the owner lived in the property at the same time as the tenant, a rule in place since 6 April 2020.

Frequently asked questions

Can I live in my annex and let the main house?

Planning conditions usually tie the annex to the main house, not the other way round, so swapping is often possible without a new permission when your household uses both. Letting the main house to a tenant while you live in the annex creates two separate households, which may breach an ancillary condition. Check the decision notice and, if in doubt, apply for a Lawful Development Certificate first.

Does an annex add value to a house?

An annex with full planning permission and building regulations sign-off typically adds value, because it suits multi-generational buyers and home workers. An annex built without consents can do the opposite, since buyers' solicitors will raise it and lenders may refuse to lend against it.

Can I sell an annex separately from the main house?

Only with planning permission for a separate dwelling. Most annexes are approved as ancillary to the main house, and selling one off would mean creating a new home, which needs its own permission, address and title.

Is a let annex an HMO?

No, not on its own. An annex let to a single household is a self-contained dwelling. It becomes an HMO only if three or more people from more than one household live there and share facilities.