Measure rear depth from the original wall
For a rear extension, the government's technical guidance measures the projection from the base of the relevant original rear wall to the outside wall of the enlargement. Roof overhangs and gutters are not the depth reference described in that guidance. A later extension does not create a fresh original rear wall. [1]
A stepped rear elevation needs particular care. Where the original house includes an outrigger or several rear-facing wall sections, show each relevant wall on the plans and test the proposal against the guidance. Do not automatically use the furthest rearward point of the house as a single starting line for everything. [1]
Make the dimensions on existing and proposed plans comparable. Label which wall is original and identify existing additions which will remain, be altered or be removed. This is a practical way of making the required assessment visible rather than leaving it to inference.
Height is measured from adjacent ground
The general height reference is ground level immediately adjacent to the building. Where that ground is not uniform, the technical guidance uses the highest part of the adjacent ground surface. A raised deck is not the ground-level datum. [1]
On a sloping site, record the levels beside the proposed building and show them on the elevations or sections. This avoids presenting a height dimension without explaining which ground point it starts from. The relevant rule must still be checked for the specific development class. [1]
Eaves height and overall height are different
The technical guide explains eaves by the meeting of the external wall with the upper surface of the roof, projected where necessary. A parapet or gutter should not simply be labelled “eaves” without applying that definition. A separate overall-height limit can still control the highest part of the extension. [1]
For house extensions under Class A, being within 2 metres of a curtilage boundary brings a 3-metre eaves-height limit. That is not a general permission for every building near a boundary to be 3 metres high. Outbuildings have different rules, including a 2.5-metre overall-height limit when within 2 metres of the boundary under the relevant householder right. [2][1]
Check the boundary and the area being counted
“Curtilage” is a planning term for land associated with the house; the technical guidance notes that it may be less than the whole property grounds. Coverage calculations exclude the original house from the relevant comparison but include other buildings and additions as the rules specify. [1]
Where the boundary position is uncertain, flag it before using a narrow dimensional margin. A planning certificate cannot resolve private ownership rights merely by showing a line on a drawing. [3]
A useful measurement schedule
Record the original rear-wall references, extension depth, overall height, eaves height, ground levels, boundary distances and the buildings included in any coverage calculation. Add width and roof-volume calculations where the particular class requires them. Each dimension should be traceable to the same drawing revision. [1]
Measurements establish only part of compliance. Property eligibility, planning conditions, Article 4 directions and any required prior approval must still be checked. [4][5]