Cornwall Council’s annexe guidance treats an annexe as accommodation that remains ancillary to a principal dwelling. A self-contained layout does not by itself establish that the proposal qualifies as an annexe or can be handled through a householder application. The planning authority assesses the actual use, site relationship, and proposal.
Show the relationship to the principal dwelling
The Council says an annexe should have a functional link to the main home, remain in the same ownership, sit within the same curtilage, share vehicular access, and be well related to the principal dwelling. It may be an extension, a detached building, or a conversion of an existing building. The Council advises applicants to explain who will occupy it, their relationship to the main household, shared use of the main house, and how the proposal meets the ancillary criteria.
For countryside sites, the Council says an extension should be considered where possible. A separate building needs clear justification. Its guidance says, as a guide, a detached annexe should not exceed 50% of the existing dwelling’s footprint and should be capable of incorporation into the main dwelling if the annexe is no longer needed. Proposals with more than one bedroom will normally be refused unless there is a specific and compelling reason for the additional bedroom.
Identify the planning application route
Cornwall distinguishes an extension ancillary to an existing dwelling from a self-contained outbuilding. Its guidance says a self-contained outbuilding with bedroom, living room, bathroom, and kitchen is not a householder application and uses the full planning-application route. A decision may also include a condition that prevents the building from being occupied as a separate residential unit. The Council notes that proposals differ and that it may request further information.
Confirm the current local decision
This is Cornwall-specific planning guidance, not a rule for every authority in England or the UK. Confirm the current development plan, site designations, the proposal’s use and layout, and any separate building-regulations or heritage requirements with Cornwall Council. The guidance itself was last updated April 1, 2026; any application fee examples are excluded here because they may change.