Will an annexe mean a second council tax bill?

It can. Annexes are sometimes banded separately, though relief may apply where a dependent relative occupies it. It is worth asking the council directly at design stage, because the answer occasionally changes what people want to build.

Can we let the annexe out later?

Not if it carries an ancillary occupancy condition, and that condition is common. If independent letting is the real goal, the honest route is to apply for a separate dwelling and accept that it is a harder application.

Is a garden room enough?

Only for occasional use. A garden building used as permanent sleeping accommodation is a material change of use and needs both planning consent and building regulations approval — a supplier's claim that it is permitted development does not survive that.

Does an annexe add value?

We would not put a figure on it, and neither should anyone else without valuing your specific property. What we can say is that a well-designed annexe with proper separation is a different proposition to a bedroom with a kettle in it.

Project type · Cornwall

Annexes and granny flats in Cornwall — ancillary means ancillary, and that word decides the application

Multi-generational living is now one of the most common briefs we take in Cornwall. The planning system treats an annexe very differently from an independent dwelling, and almost every refusal turns on the same point: whether what has been drawn is genuinely ancillary to the main house or is, in practice, a separate home.

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What makes an annexe ancillary

There is no single test, but officers look at a consistent set of indicators. Designing with these in mind from the first sketch is far easier than arguing about them after a refusal.

Physical or functional connection to the main house rather than a wholly detached, self-contained unit
Shared access, parking and garden rather than a separately fenced curtilage
Scale proportionate to the main dwelling
Willingness to accept a condition tying occupation to the main house

Attached, linked or detached

Attached annexes — a converted garage, a rear extension with its own bedroom and shower room — are usually the smoothest route. A glazed link is a well-established way of keeping the connection while giving both households acoustic separation. A fully detached garden building is the hardest case, and in the countryside or the AONB it is often the one that fails.

Regulations, fire and independence

Whatever the planning route, the annexe has to work as accommodation: escape windows or protected routes, insulation and ventilation to current standards, sensible acoustic separation between households, and heating that can be controlled independently so an older relative is not paying to warm the whole house.

Frequently asked

Straight answers.

Will an annexe mean a second council tax bill?: It can. Annexes are sometimes banded separately, though relief may apply where a dependent relative occupies it. It is worth asking the council directly at design stage, because the answer occasionally changes what people want to build.
Can we let the annexe out later?: Not if it carries an ancillary occupancy condition, and that condition is common. If independent letting is the real goal, the honest route is to apply for a separate dwelling and accept that it is a harder application.
Is a garden room enough?: Only for occasional use. A garden building used as permanent sleeping accommodation is a material change of use and needs both planning consent and building regulations approval — a supplier's claim that it is permitted development does not survive that.
Does an annexe add value?: We would not put a figure on it, and neither should anyone else without valuing your specific property. What we can say is that a well-designed annexe with proper separation is a different proposition to a bedroom with a kettle in it.

Planning an annexe for family?

Tell us who it is for and how independent it needs to be — that shapes the whole application.

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