An annex – or “annexe” in British English – is a secondary living space attached to, or built within the grounds of, a main home. It’s self-contained, with its own bedroom, kitchen and bathroom, but stays legally and functionally tied to the main property, sharing access and services rather than operating as a fully independent dwelling. In the UK, annexes are most commonly used to house elderly parents, adult children saving for their first home, or paying guests, and they’re often called a “granny annexe” or “granny flat” as a result.

What is an annexe building?

The word “annex” gets used loosely, so it helps to see it next to the buildings people usually confuse it with. We’ll look at the differences between annexes, granny annexes, garden rooms, extensions and caravans or mobile homes.

A granny annexe is simply an annexe built specifically to house a relative – the same building, with a specific use in mind. A garden room, by contrast, isn’t designed or legally permitted to be lived in full-time; it’s built under permitted development rules for occasional use, like an office or gym. 

A house extension achieves the same living space as an annexe but is structurally part of the main house rather than a separate or semi-separate building. And a static caravan or “mobile home” style annexe is a specific legal category with its own rules, covered in more detail below.

Type	Can you live in it full-time?	Needs planning permission?	Structure
Annex / Annexe	Yes	Usually	Attached to the house or a separate garden building
Granny annexe	Yes (for a relative)	Usually	Same as above
Garden room	No – occasional use only	Usually not (permitted development)	Detached garden building
House extension	Yes	Usually	Structural extension of the main house
Static caravan / mobile home	Yes, if it meets the legal test	Sometimes avoidable via a Lawful Development Certificate	Manufactured off-site, must meet size/construction/mobility tests

Attached vs detached annexes

An attached annexe shares a wall (and usually shares utilities) with the main house. Local planning authorities often expect its materials and appearance – brickwork, roof tiles, window style – to match the existing house.

A detached annexe sits as its own building in the garden. It can cost more to connect to mains services, but it doesn’t need to match the main house’s appearance, gives everyone more privacy, and is the more common choice for a genuinely self-contained granny annexe. 

This is exactly where high-quality annexe specialists come in – companies like Summerhouse24, which design and manufacture log cabin-style annexe buildings, are set up specifically to deliver this kind of detached, fully self-contained build.

Ready to see what a granny annexe could look like in your garden?

When is an annexe “ancillary” vs a separate dwelling?

Planning authorities allow annexes because they’re treated as “ancillary” to the main house – existing to support the household, not to function as an independent home in their own right. 

Where that line sits is the single most important (and most commonly misunderstood) thing about annexes, because crossing it changes your planning position, your council tax, and what you’re legally allowed to do with the building.

Curtilage, explained properly

Annexes are generally expected to sit within the curtilage of the main property. Curtilage isn’t the same as “your whole garden” – courts have defined it as the land you actively use and maintain immediately around the house: the lawn, the patio, flower beds, and similar. 

If you have a larger plot with a paddock or orchard towards the back, that area may well sit outside your curtilage. If you want to site an annexe there, you may need a separate application to extend the curtilage before you can even apply to build.

What “functional link” actually means

You’ll often see planning guidance say an annexe must have a “functional link” to the main house. In practice, this means the annexe shares access and services – water, power, drainage – with the main dwelling, rather than operating as a fully self-sufficient property. It’s one of the clearest tests planning officers use to distinguish an annexe from a new, independent house.

Beyond curtilage and functional link, planning authorities also weigh: 

• the annex’s size relative to the main house (one that rivals the house in floor area invites more scrutiny), 
• how fully equipped it is (a bedroom with an en suite reads very differently to a full kitchen, multiple bedrooms and its own garden area), 
• and whether it could realistically be sold or let independently in future, even if that’s not your current intention. 

No single factor is decisive – it’s the overall picture that determines whether a proposal is treated as ancillary accommodation or a new dwelling.

Do you need planning permission for an annexe building?

In most cases, yes. As soon as you introduce primary living accommodation – somewhere someone can sleep, cook and wash – into an outbuilding, it moves out of “incidental use” and into ancillary residential use, which doesn’t benefit from permitted development rights in the way a garden office or gym does. But there’s a bit more to it.

Permitted development vs full planning permission

Permitted development rights let you build certain outbuildings without planning permission, but they come with limits – including a maximum eaves height of 2.5 metres for garden buildings – and they explicitly exclude use as sleeping accommodation. 

Once you’re building somewhere intended for someone to actually live, you’ll almost always need to apply for planning permission in the normal way. For a full walkthrough of the process, see our granny annexe planning permission guide.

It’s worth reassuring yourself that this is far more manageable than it used to be. 

Official government figures show that in the year to March 2026, local authorities granted 90% of householder planning applications, and decided 93% of them within eight weeks or the agreed timeframe. 

Between the Planning Portal’s online application system and the free pre-application advice most councils now offer, getting consent for a well-designed annexe is a genuinely achievable process rather than the ordeal people often expect it to be.

The Caravan Sites Act route

There’s a lesser-known alternative some annexe providers use: building to the legal definition of a “mobile home” or “caravan” under the Caravan Sites Act, which can sometimes be sited under a Lawful Development Certificate rather than full planning permission. 

To qualify, a structure generally has to pass three tests: 

• size (no wider than 6.8 metres, no longer than 20 metres, with an internal height of up to roughly 3.05 metres), 
• construction (built in a way that demonstrably could be assembled from transportable sections, rather than built brick by brick on site), 
• and mobility – which doesn’t mean it needs wheels or a tow bar, just that it’s capable, in principle, of being moved. 

This route still requires the annexe to sit within your curtilage, and it’s worth getting proper planning advice before assuming it applies to your project. We cover this route in much more depth in our granny annexe Caravan Act guide.

Checking site constraints before you design

Before finalising a design, it’s worth checking for anything that could complicate an application: 

whether the property is in the green belt, an area with flood risk, within the curtilage of a listed building, or in an Area of Outstanding Natural Beauty. Flagging these early can shape the design and save a rejected application further down the line.

Will you pay council tax on an annexe?

Usually, yes, though there are meaningful reductions available depending on who lives there – see our full council tax guide for granny annexes for the details.

The self-contained-unit test

The Valuation Office Agency (VOA) decides whether an annexe gets its own council tax band. The key test is whether it counts as a self-contained unit – broadly, does it have its own kitchen, bathroom and bedroom – even if it shares an entrance, water or heating with the main house. If it qualifies, it can be banded separately from the main property.

50% discount vs full exemption

If a family member lives in the annexe, you can typically apply for a 50% discount on the separate council tax charge. If that person is a dependent relative – for example, over 65, disabled, or dependent on the household for care – and doesn’t have another home of their own, the annexe may qualify for a full exemption instead. 

Rules and application processes vary by council, so it’s worth contacting your local authority directly to confirm what applies to your situation.

Challenging a VOA banding decision

If you think your annexe has been banded incorrectly, you can formally challenge the VOA’s decision, though there are time limits for doing so – check the current guidance on GOV.UK or speak to your local council before the window closes.

Does an annexe add value to your property?

Yes, though how much depends heavily on where you live. Regional analysis by estate agency eXp UK put a number on that variation:

Region	Average uplift	Average cash premium
Yorkshire & the Humber	24.3%	£110,500
West Midlands	23.9%	£135,095
North East	23.9%	£101,252
South East	21.4%	£167,500 (largest cash premium)
London	10.3%	£107,550

The pattern is worth noting: the South East has the smallest percentage uplift of the top three but the biggest cash gain, simply because property values there start higher. London sits at the opposite extreme – its percentage uplift is the lowest of the five, yet the cash premium is still substantial for the same reason.

eXp UK’s head Adam Day puts the demand down to flexibility. An annexe lets a household get the benefits of shared living, extra support, company, or rental potential, without either generation giving up their own space, and buyers are clearly willing to pay for that option. 

It’s also become a genuine lifestyle solution as more UK households look for ways to support ageing parents or adult children without everyone living under one roof full stop – and any uplift still needs to be weighed against the build, planning and (potentially) council tax costs involved. 

Can you sell or rent an annexe?

This comes up constantly, and the honest answer is: it’s more restricted than most people expect.

Selling: because an annexe is legally ancillary to the main house and normally sits on the same title and address, you generally can’t sell it as a separate property. 

Planning consent for an annexe is often granted with a condition specifically preventing it from being sold or occupied independently of the main house. In practice, an annexe adds value to the whole property rather than becoming a separate saleable asset in its own right. It’s sometimes possible to apply to have that condition removed, but councils consider this case by case, it isn’t guaranteed, and it effectively asks the council to accept the creation of a genuinely new, independent dwelling.

Renting to family or a lodger: having a relative live there rent-free or for a contribution towards bills is generally straightforward. A lodger arrangement – where the person shares facilities with you as the householder – can also work, but if the annexe has its own kitchen and the occupant doesn’t share facilities with you, they may legally count as a tenant rather than a lodger, which can change your legal position.

Renting to a non-family tenant: letting the annexe out independently, on a standard tenancy, usually requires a planning application for change of use, because it shifts the building from ancillary accommodation to an independent dwelling. It can also trigger a separate council tax band and may need sign-off from your mortgage lender and home insurer.

Short-term or holiday letting: renting an annexe out on a platform like Airbnb is generally treated as a business use. It typically needs its own planning permission, can affect your council tax or business rates position, and again may require you to update your mortgage and insurance arrangements. If you’re not making significant income from it, it’s worth checking whether you qualify for Small Business Rate Relief, which can reduce or even eliminate the tax liability on a modest holiday let altogether.

Because enforcement and interpretation vary between councils, always check with your local planning authority before selling, letting, or renting out an annexe – and treat this section as general guidance rather than legal advice specific to your property.

Building regulations – what an annexe must comply with

Whichever planning route you take, an annexe intended for habitation still has to meet Building Regulations – covering structural safety, fire safety, drainage, and Part L, which governs insulation and ventilation standards. Part L sets maximum U-values (a measure of how much heat a building element loses) for walls, floors and roofs, and every registered installer should be able to show how their build complies. Because these standards are periodically updated, check the current Approved Document L requirements – or ask your installer to confirm compliance – rather than relying on older figures.

FAQs

Explore annexe options

Thinking about adding a granny annexe to your garden? Explore Summerhouse24’s range of self-contained annexe buildings, or read our full Granny Annexe Guide for more on cost, planning permission and council tax.

---

Sources used:

• legislation.gov.uk – Caravan Sites Act 1968 
• GOV.UK – Permitted development rights for householders: technical guidance 
• GOV.UK – Valuation Office Agency 
• GOV.UK – ​​Planning applications in England: statistical release 
• Property Portfolio Investor – Adding an annexe increases house value – eXp UK research
• GOV.UK – Small Business Rate Relief
• GOV.UK – Conservation of fuel and power: Approved Document L
• GOV.UK – Planning applications in England: January to March 2026 – statistical release