Annexes and the single planning unit question

An annexe needs planning permission when it creates a separate dwelling, and it does not when it remains part of the same household. That single distinction, known in planning terms as the planning unit question, decides whether your granny annexe is an alteration to an existing house or a new home in the garden. The physical building can be identical either way. What changes is how it is used, how it relates to the main house and what the council can enforce afterwards.

Most refusals in this area are not about design. They are about a council concluding that what is proposed is, in substance, a second dwelling on a plot that policy would not allow one on.

Ancillary use against a separate dwelling

Section 55 of the Town and Country Planning Act 1990 makes a material change of use development. Creating an additional dwelling is a material change of use. Using part of your house, or an outbuilding within its curtilage, for purposes ancillary to that house is not.

So an elderly parent living in a converted garage, eating with the family, sharing the garden and the bins, and coming in and out through the same gate, is part of one dwelling. A tenant paying rent to live in the same converted garage, with their own front door, their own postbox and no functional connection to the house, is occupying a second dwelling.

Courts have described the distinctive characteristic of a dwellinghouse as a building that ordinarily provides the facilities needed for day to day private domestic existence. That sets the bar for what could be a separate dwelling. It does not answer whether it is one, which is where the planning unit comes in.

What makes a separate planning unit

The tests come from case law rather than statute, and the leading formulation asks whether the whole unit of occupation is used for a single main purpose, with other activities incidental to it, or whether physically separate and distinct areas within it are occupied for substantially different and unrelated purposes.

Applied to an annexe, officers look at a familiar cluster of factors:

• Physical separation: attached to the house, linked by a corridor, or a free standing building across the garden.
• Self-containment: whether the accommodation could function without using any part of the main house.
• Access: a shared front path and gate, or an independent entrance from the street.
• Services: shared meters, heating and drainage, or separate supplies.
• Curtilage: a shared garden with no dividing line, or a fenced private garden with its own bin store and parking space.
• Occupation: a member of the household, or an unconnected occupier.

No single factor decides it. A separate entrance on its own is unremarkable. A separate entrance, a fenced garden, its own meters and a lettings history together describe a second dwelling whatever the application form says. The closer analysis of exactly where that line falls is set out in annexe or separate dwelling: where the line falls.

Kitchens, entrances and shared facilities

Officers ask about kitchens because a kitchen makes independent living possible, and they ask about entrances because an entrance makes it practical.

A full kitchen does not by itself create a separate dwelling, and councils that insist on removing one are applying a rule of thumb rather than law. But the combination of a full kitchen, a bathroom, a separate entrance and a lockable internal door between annexe and house leaves very little factual link for an applicant to rely on later.

The design moves that keep an annexe clearly ancillary are modest. Retain an internal connection to the house that is used in practice rather than only drawn. Share the garden rather than subdividing it. Share the drive and the bin store. Keep the utility connections on the house supply. Where an internal link is impossible because the annexe is a detached outbuilding, the functional links become the garden, the parking and the services, so do not fence any of them off.

Why a garden annexe is rarely permitted development

Class E of Part 1 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015 allows outbuildings within the curtilage of a house, but only for purposes incidental to the enjoyment of the dwellinghouse.

Incidental is narrower than ancillary. Courts have held that primary living accommodation is not incidental to a house, because sleeping and living are the house's main purpose rather than something ancillary to it, and a home office, gym, store or garden room sits comfortably inside Class E while a bedroom and bathroom for a relative does not.

That is why most garden annexes need a planning application even when they are small enough to satisfy every dimension in Class E. The same reasoning applies in reverse to conversions inside the house, which are usually outside the definition of development altogether unless a new dwelling is created, as with subdividing a house into two flats.

Conditions that tie an annexe to the house

Where permission is granted, expect a condition. The standard form restricts occupation of the annexe to use ancillary to the main dwelling and prohibits its use, sale or letting as a separate dwelling.

That condition is generally enforceable and generally reasonable, and it is worth offering in the application rather than waiting to receive it, because an applicant proposed condition is usually tighter drafted than one written under time pressure. Offer it with a plan that shows the shared curtilage, so the extent of what is tied is clear.

Some councils go further and restrict occupation to a named relative or a dependent. Those conditions are harder to justify, because they control who lives somewhere rather than how land is used, and they create problems when circumstances change. If one is proposed, ask for the ancillary use wording instead.

Council tax, the levy and what they do not decide

A self-contained annexe can be given its own council tax band by the Valuation Office Agency, and separate banding often arrives unprompted after a completion certificate. It is evidence that the accommodation is self-contained. It is not a finding that the annexe is a separate planning unit, and the two regimes apply different tests.

There is a council tax reduction for annexes occupied by a relative of the household, so check the current position with the billing authority rather than assuming a full second bill. Separately, a residential annexe within the curtilage of a main dwelling can qualify for exemption from the Community Infrastructure Levy, but the claim has to be made and acknowledged before works start. Miss that sequence and the exemption is lost, whatever the merits.

Selling or letting the annexe later

This is the question people ask five years afterwards, and the answer is usually no, not without a further application.

Where a condition ties the annexe to the main house, selling it separately breaches the condition, and a breach of condition can be enforced against. Where there is no condition because the annexe was built as part of the house, separate occupation is a material change of use in its own right and needs permission. Either way the route is an application, supported by a Planning Statement that addresses the policy tests for a new dwelling on that plot: access, parking, amenity space, privacy and whether the location is one where a new home is acceptable at all.

Time limits are not a reliable route. Enforcement time limits for breach of condition and for change of use exist, but the periods have been changed by recent legislation with the dates on which they take effect to be verified, and relying on immunity means relying on nobody noticing for years. A lawful development certificate under section 191 of the 1990 Act is the only way to establish that an existing use has become lawful.

What to submit

For a straightforward annexe application the pack is short: the application forms, a location plan, a site plan showing the shared drive, garden and bin store, existing and proposed floor plans and elevations, and a short statement.

The statement is the part that does the work. It should say who will occupy the annexe and in what relationship to the household, describe the functional links to the main house, identify the internal connection if there is one, confirm that the curtilage is not being subdivided, and offer the ancillary use condition. Where the house is listed or in a conservation area, a Design and Access Statement is likely to be required as well.

This is general guidance, not legal advice; the planning unit question turns on the facts of each case, so check the position with your local planning authority or a qualified planning professional before relying on it. A Lawful Development Certificate is the only formal way to confirm that a particular use is lawful.

Enter your postcode on PlanScope to see which documents an annexe application at your address is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.