Annexe or separate dwelling: where the line falls

An annexe stays ancillary to your house while the household uses it as part of the same home. It stops being ancillary the moment it could run on its own. That distinction, annexe or separate dwelling, decides whether you have carried out development at all, and floor area has very little to do with it. A 20 m2 garden room with a shower and a hob can be a separate unit. A 90 m2 wing of the house with its own sitting room may not be.

Section 55(3)(a) of the Town and Country Planning Act 1990 says that using a building previously occupied as a single dwellinghouse as two or more separate dwellinghouses is a material change of use. So the question a case officer asks is never whether the annexe is small enough. It is whether the property is still one planning unit or has quietly become two.

What makes accommodation ancillary

Two words get muddled constantly, and the difference matters. Incidental use covers the things that go with occupying a house rather than living in it: a home office, a garage, a workshop, a gym. Ancillary use is wider and covers living accommodation that belongs to the main household, such as a bedroom and bathroom for an elderly parent who eats with the family.

That matters because permitted development for outbuildings under Class E of Schedule 2 Part 1 of the General Permitted Development Order 2015 is limited to purposes incidental to the enjoyment of the dwellinghouse. Primary living accommodation falls outside it. Building a garden annexe with a bedroom, kitchen and bathroom under Class E and calling it a home office is the single most common way householders end up on the wrong side of an enforcement notice.

Accommodation inside the house, or attached to it under Class A, is different. An extension containing a bedroom and shower room for a relative does not create a second dwelling on its own. The use has to change as well.

Shared facilities, access and utilities

Officers test independence through physical facts, because those are the facts an enforcement officer can stand in a garden and observe. The usual list runs like this.

• Whether the annexe has its own cooking facilities, and whether they are a full kitchen or a kettle and microwave
• Whether there is internal access between the annexe and the main house, and whether that door is usable or blocked by furniture
• Whether the annexe has a separate postal address, its own front gate, its own bin store or its own driveway space
• Whether utilities are sub-metered or separately supplied, and who pays the bills
• Whether the garden is divided by a fence or hedge, or shared

None of these is decisive by itself. An annexe with a small kitchenette and a shared garden, reached through the family's back door, reads as ancillary. The same building with a separate meter, a fenced yard and a gate onto the lane reads as a flat. Judgement sits in the middle of that range, and it is worth designing the scheme so it sits obviously at one end.

The planning unit and where its edges sit

The planning unit is the parcel of land in one occupation to which a primary use applies. The approach still used by inspectors comes from Burdle (1972), which asks whether the whole site is in single occupation with one main purpose, whether there are physically separate areas each with their own use, or whether an area within a larger unit has a use that is not ancillary to the main one.

Applied to an annexe, the test asks who is actually in occupation. A relative living rent free, sharing meals, laundry and the garden, sits inside the household. A tenant paying rent on an assured shorthold tenancy, with exclusive possession of a self-contained unit, does not. The occupier's legal interest can move the planning unit even when nothing physical has changed.

This is why "we will only ever have family in it" carries weight at application stage and then evaporates when the family moves out. Councils know that. It is why they reach for conditions.

Conditions that tie occupation to the main house

Where an annexe is approved as ancillary, expect a condition roughly to the effect that the accommodation shall be used solely for purposes ancillary to the dwelling and shall not be occupied as a separate unit of residential accommodation. Some authorities go further and name the relationship, restricting occupation to a dependent relative of the occupier of the main house.

Conditions have to meet the six tests set out in national policy and explained in the Planning Practice Guidance on the use of planning conditions: necessary, relevant to planning, relevant to the development, enforceable, precise and reasonable. A condition tying an annexe to the main house is normally all six. A condition naming an individual person rarely survives an appeal, because it is not enforceable against a successor.

Read the condition before you accept the decision notice. If it restricts occupation to a named relative, ask for it to be reworded to the ancillary formula instead. That conversation is far easier before the decision is issued than after.

Council tax, and why it is not the planning test

Annexes are often banded separately for council tax by the Valuation Office Agency once they are self-contained. Owners then assume the council has accepted a second dwelling. It has not. Council tax banding runs under the Local Government Finance Act 1992 and asks a different question, and the two regimes reach different answers all the time.

The banding does matter as evidence. If an enforcement officer sees a separate council tax band, a separate electricity account and a tenancy agreement, that bundle tells a story about how the building is used. Keep the planning position and the practical position aligned, or be ready to explain the gap.

Community Infrastructure Levy works the other way round. The CIL Regulations 2010 provide an exemption for a residential annexe built within the curtilage of a main dwelling, subject to claiming it before work starts and to a clawback period. Charging schedules and the claim forms change, so check the current position with the charging authority before you start work. Miss the claim and the exemption goes with it.

Selling or letting the annexe later

An annexe cannot be sold separately while it remains part of one planning unit, and an ancillary occupancy condition will block a lender's solicitor cold. The route to independence is a planning application for change of use to create a separate dwelling, or an application under section 73 to vary the condition.

That application is a real test, not a formality. The council will look at amenity standards, private outdoor space, parking, refuse storage and whether the resulting unit meets the nationally described space standard where the authority has adopted it. Biodiversity net gain, affordable housing and CIL may all engage. A supporting Planning Statement that deals with those points directly is worth more than plans alone, and where the annexe reads as a subservient addition the same design arguments apply as in a two storey side extension.

If the annexe sits close to mature trees, root protection areas will shape both the footprint and the construction method, which is a separate conversation covered in our note on extensions near trees.

Confirming the position formally

A lawful development certificate is the only way to put the question beyond argument. Under section 192 of the 1990 Act you can apply for a certificate that a proposed use or operation would be lawful. Under section 191 you can apply in respect of an existing use. The fee for a proposed use certificate is half the equivalent planning application fee, and the current schedule is published by government.

The application succeeds or fails on evidence, not on planning merits. Floor plans showing the internal connecting door, a statement of how the accommodation will be occupied, utility arrangements and site plans showing the shared garden all carry weight. A Design and Access Statement is not required for a certificate, though the drawings still need to be accurate to the millimetre, because the certificate only protects what is described in it.

Illustrative example. A couple convert an integral garage and a single storey rear projection on a 1930s semi into a bedroom, shower room and sitting area for a parent, keeping a connecting door to the hall, no separate kitchen and one shared front path. On those facts an officer would usually treat the accommodation as ancillary. Change one element, add a full kitchen and brick up the connecting door, and the same building starts to look like a flat. The example is illustrative only and your council may weigh the facts differently.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Enter your postcode on PlanScope to see which reports your annexe proposal is likely to need.