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Annexe Calculator — Self-Contained Requirements & Planning
Assess annexe self-contained requirements including kitchen, bathroom and separate entrance, plus Council Tax implications and planning considerations.
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How We Calculate This
This calculator assesses annexe requirements based on configuration, self-contained status and compliance needs.
Self-contained assessment
An annexe with a kitchen, bathroom and separate entrance is self-contained. This triggers potential separate Council Tax and affects planning requirements.
Space standards
Minimum gross internal floor areas follow the Nationally Described Space Standard (Table 1, single-storey): 39m² (1-bed/1-person; 37m² only with a shower room not a bathroom), 50m² (1-bed/2-person), 61m² (2-bed/3-person), 74m² (3-bed/4-person) and 90m² (4-bed/5-person). The standard also requires a ceiling height of at least 2.3m over at least 75% of the floor area, but sets no minimum kitchen-only or bathroom area.
Frequently Asked Questions
A self-contained annexe has its own kitchen (with cooking facilities, sink and food storage), bathroom (WC, basin, bath or shower) and a separate entrance that does not require going through the main house. Self-contained annexes may attract separate Council Tax and require planning permission as a separate dwelling unit.
Possibly. If the annexe is self-contained, the Valuation Office Agency may assess it for separate Council Tax. However, if it is occupied by a relative, many councils apply a 50% discount on the annexe assessment. Non-self-contained annexes are typically included in the main property assessment. Contact your local council before building.
In most cases, yes. A new build annexe or converting an outbuilding to self-contained living space requires planning permission. The annexe must be ancillary to the main dwelling — it cannot be used as a separate, independent dwelling or holiday let without further planning consent.
Under the Nationally Described Space Standard (2015, which applies only where a local plan policy adopts it), a one-bedroom one-person single-storey dwelling should be at least 39m² gross internal area (this reduces to 37m² only where it has a shower room instead of a bathroom); a one-bedroom two-person dwelling is at least 50m². A two-bedroom (2-bed/3-person) dwelling should be at least 61m², a three-bed (3-bed/4-person) at least 74m² and a four-bed (4-bed/5-person) at least 90m². The NDSS requires a floor-to-ceiling height of at least 2.3m over at least 75% of the gross internal area. It sets no minimum kitchen-only or bathroom area. Your local authority may have additional requirements.
Not without planning permission for a change of use. An annexe granted planning permission as ancillary to the main dwelling must remain ancillary — typically occupied by a family member or dependent. Renting to a non-family member would constitute a change of use requiring separate planning consent. Short-term holiday letting may also need planning permission.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.