Annexes and outbuildings as a separate dwelling: when it stops being permitted development
The line between an "incidental" outbuilding and a genuine separate dwelling decides planning permission, council tax, and — as the recent BNG changes showed — biodiversity net gain exposure. Here's where that line actually sits.
Three separate parts of officialdom — planning, council tax, and now biodiversity net gain — all ask a version of the same question about a garden annexe: is this genuinely part of the main house, or is it actually a separate dwelling that happens to sit in the same garden. The answer to that question, not what the building is called on the drawings, decides all three.
What actually makes something 'a dwelling'
The tests planning officers and, separately, the Valuation Office Agency for council tax purposes apply overlap without being identical. Independent facilities — a kitchen, bathroom, sleeping space — a separate entrance not shared with or dependent on the main house, and no functional connection requiring routine access through the main dwelling, all point toward 'separate dwelling' rather than 'annexe incidental to the house'.
Why 'granny annexe' framing doesn't settle the planning question
Calling something an annexe for an elderly relative describes an intended use, not a planning category. If it has full independent facilities and could function as a separate household regardless of who's actually living there, it can be treated as a new dwelling for planning purposes whatever it's called. A condition restricting occupancy — tying it to the main house, or to a family member — is often what actually delivers the 'annexe' outcome the applicant wants, rather than the design alone doing that work.
Permitted development stops applying at exactly this point
As covered separately, outbuilding permitted development rights require the building to be incidental to the enjoyment of the house. A structure that reads as a genuine separate dwelling falls outside that regime by definition, regardless of its size, and needs a full planning application rather than proceeding as permitted development.
Council tax: a separate assessment, potentially a separate band
An annexe that qualifies for its own council tax band, rather than being included within the main house's assessment, is — by that same underlying logic — being treated by a different public body as a separate dwelling. Worth checking how this interacts with the planning position rather than assuming the two are unrelated, since a favourable council tax discount for an annexe and a planning argument that it's 'incidental' can end up pulling in opposite directions.
Biodiversity net gain: the connection worth restating
The BNG changes covered elsewhere on this site apply the same underlying test a third time. An annexe assessed as a separate dwelling is judged against the general small sites exemption on its own terms — 0.2 hectares, no priority habitat — rather than being waved through under a householder exemption that only ever covered genuinely incidental structures.
Getting the occupancy condition right, if that's the intended outcome
If the genuine intention is a dependent relative's annexe rather than an independent letting unit, a planning condition explicitly restricting occupation to a family member of the main house, and requiring the annexe to be used together with rather than separately from the main dwelling, is usually the mechanism that makes an otherwise borderline scheme acceptable. Worth proposing this proactively in the application rather than waiting for it to be imposed later, or for the application to be refused for lacking it.