The Peak District published a comprehensive policy document on the complexity of determining what is ancillary and what is not. Link below

Peak District – Residential Annexes – Supplementary Planning Document – Adopted Feb 2021

Residential annexes is a complex area of planning, largely due to the high level of Case Law involved. It is therefore advised that those seeking to erect a new build residential annex or convert an existing building into a residential annex should discuss their proposals with the Authority via the pre-application advice service prior to submitting a planning application or starting work.

In determining a planning application for ancillary residential accommodation (either by conversion or new build) an authority will expect the proposed development to:

• be subordinate in scale to the main dwelling house
• share a vehicular access with the main dwelling house
• be in the same ownership as the main dwelling house
• be located within the residential curtilage or building group associated with the main dwelling house
• have a functional connection/degree of dependence with the main dwelling house (e.g. the occupant should be a dependent relative of the residents of the main dwelling house, a carer or be employed at the main dwelling as an au pair, servant, nanny etc.);
• If more than one bedroom is proposed within the ancillary residential accommodation, clear justification must be provided as to why each bedroom is required.
• Planning applications should always be accompanied by supporting information clearly setting out the justification for the proposed development, including who the intended occupant(s) of the ancillary residential annex will be.

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5.2 In determining a planning application for ancillary residential accommodation (either by conversion or new build), the Authority will expect the proposed development to:
 be subordinate in scale to the main dwelling house in the case of new development;
 share a vehicular access with the main dwelling house;
 be in the same ownership as the main dwelling house;
 share utilities with the main dwelling house;
 be located within the residential curtilage or building group associated with the main dwelling house as well as within the same planning unit
 be sited so as not to have a detrimental impact on:
 valued landscape character and/or
 cultural heritage significance as defined in the Landscape Strategy
 Conservation Area appraisals
 Farmstead Heritage Assessments
 Non designated heritage assets as determined by the Authority in line with Historic England guidance for buildings not currently recognised as heritage assets or neighbouring amenity;
 have a functional connection/degree of dependence with the main dwelling house (e.g. the occupant should be a dependent relative of the residents of the main dwelling house, a carer or be employed at the main dwelling as an au pair, servant, nanny etc.);
 contain a level and scale of accommodation that can be justified for its intended occupants;
 have no boundary demarcation or sub-division of the garden areas between the main dwelling house and the annex;
 conserve and enhance the heritage significance/setting of:
 the existing building/building group
 main dwelling house
 Conservation Area
 Listed Building
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And, where applicable, also;
 comply with the Authority’s design standards
 respect neighbour amenity

5.3 Planning applications should always be accompanied by supporting information clearly setting out the justification for the proposed development, including who the intended occupant(s) of the ancillary residential annex will be.