Walkthrough: an annexe that became a separate dwelling

An annexe stops being an annexe the moment it is occupied independently of the main house. Not when the kitchen goes in, not when the council tax band changes, but when the way the buildings are used means the house and the annexe are no longer one planning unit. The walkthrough below is illustrative rather than a real case, and it follows a fairly ordinary garden annexe from construction to a tying condition, because the sequence repeats across the country with the same turning points.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

What was built and how it was used

The illustrative scheme is a single storey outbuilding in the rear garden of a 1960s detached house: 42 square metres, a bedroom, a shower room, a kitchenette and a living space, built under permitted development as an incidental outbuilding for an elderly parent.

For four years it worked as intended. The parent ate most meals in the main house, shared the washing machine, used the same drive and the same bins.

Then the parent moved into residential care and the owners let the annexe to a tenant found through a listing site. The tenant had their own key to a new side gate, paid rent monthly, had no connection to the household, and never entered the main house. The owners arranged separate electricity metering so the bills could be split.

Nothing was built. The use changed.

The ancillary test and where it failed

The legal question is whether the property remains a single planning unit. The approach from Burdle looks at the unit of occupation, whether parts are physically and functionally separate, and whether any activity is ancillary to the main use rather than a primary use in its own right.

In the first four years the annexe passed comfortably. Shared facilities, shared household, occupation by a family member, no separate access, one council tax assessment. The living accommodation was an extension of the household rather than a household of its own.

The letting changed every factor at once. Independent occupation by an unconnected person, a lockable separate access, separate metering, rent, and self-contained facilities covering sleeping, washing and cooking. On those facts the annexe had become a separate dwellinghouse, and creating a separate dwellinghouse is a material change of use requiring planning permission.

Two points are often misunderstood here. The council tax banding does not decide the planning question, though a separate band is evidence a council will use. And the physical specification does not decide it either, because a fully self-contained annexe lawfully occupied by a dependent relative remains ancillary.

The enforcement enquiry and the response

The neighbours noticed the gate. The council opened an enquiry and served a planning contravention notice under section 171C of the Town and Country Planning Act 1990 asking who occupied the building, on what terms, since when, and what facilities it contained.

That notice has teeth. Failure to respond within 21 days is an offence, and so is giving false information. The temptation to answer vaguely is the worst available option, because a vague answer invites a formal enforcement notice drafted on the council's assumptions rather than the facts.

The owners answered fully, took advice, and asked for the enquiry to be held while they regularised the position. Most authorities will do that where the breach is not causing harm and the owner is engaging. Enforcement is discretionary, and officers have long lists.

Retrospective application against regularisation

Two routes were open.

The first was a retrospective application under section 73A for change of use to a separate dwelling. That asks the council to approve the independent use permanently, which brings in amenity, parking, garden space, separation distances and the policy position on backland dwellings. In a suburban garden with no independent frontage, that application usually fails.

The second was to revert to ancillary use and apply for a certificate confirming the lawful position, or accept a condition tying the annexe to the house. The owners took this route: the tenancy was ended, the side gate was removed, and an application was made for a certificate of lawful proposed use for the building as ancillary accommodation.

The practical difference matters. The first route seeks a new planning permission and can be refused. The second confirms that what you intend to do needs no permission at all, and a Planning Statement is the document that explains the planning basis in either case.

Conditions tying the annexe to the main house

Where an annexe is approved by application rather than certificate, authorities almost always attach a tie. The wording follows a familiar pattern: the building shall be used only for purposes incidental or ancillary to the residential use of the host dwelling and shall not be occupied as a separate dwelling or let separately.

Some authorities add a requirement that the annexe be removed, or its kitchen facilities taken out, when it ceases to be occupied by a named class of person. That second limb is harder to enforce and has been criticised on reasonableness grounds in appeal decisions, so it is worth negotiating before the decision is issued.

A tie of the first kind is usually acceptable to an owner. It restricts letting, but it does not prevent occupation by family, guests, a carer, or the owners themselves. What it does affect is resale. A buyer's solicitor will find the condition, and the valuation will reflect a building that cannot be let independently.

The documents submitted

The certificate application was deliberately thin, because a certificate turns on fact and planning law rather than on merit:

1. A site location plan and a block plan showing the garden as a single curtilage with no internal division.
2. Existing floor plans and elevations of the annexe as built.
3. A statement setting out the intended use, the facilities shared with the main house, and the relationship of occupiers to the household.
4. Evidence that the separate access had been removed and the single council tax assessment restored.
5. A short note on the case law approach to the planning unit and why, on the facts, the use is ancillary.

Where an annexe application needs planning permission rather than a certificate, the pack is heavier and usually includes a Design and Access Statement, and a Heritage Statement if the host dwelling is listed or the site sits in a conservation area.

What a different approach at the outset would have cost

The avoidable cost here was not the enforcement enquiry. It was the eighteen months of uncertainty and the professional fees on a problem that a single conversation would have prevented.

Had the owners asked before letting, the answer would have been immediate: letting the annexe to an unconnected tenant creates a separate planning unit and needs permission that this site is unlikely to get. They would then have chosen between letting a room inside the main house, which remains within the single dwelling use, and not letting at all.

And if the intention from the start had been eventual independent occupation, the right route was a full application for a new dwelling at the design stage, when the layout could have been drawn to answer the objections rather than defend against them.

A last practical point. If you build an annexe under permitted development as an incidental outbuilding, the primary use is incidental to the enjoyment of the dwellinghouse, and sleeping accommodation sits awkwardly with that from the first day. Many owners would be better served applying for planning permission for the annexe at the outset and accepting the tying condition, rather than relying on permitted development and discovering the limits later. Related reading on the evidence side sits in the walkthrough of a tree that stopped a driveway and in a Class MA conversion refused on natural light, both of which turn on getting the question right before the work starts.

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