Two-storey and first-floor extensions must satisfy the applicable planning route and the council's assessment of their effect on neighbours and the original building. The exact test and the windows it protects must be taken from the relevant council's guidance. [1] [2]
Start with the planning route
Under Class A permitted development, an enlargement of more than one storey must not project more than 3m beyond an original rear wall or be within 7m of a boundary opposite that wall. Side extensions of more than one storey do not qualify. Additional limitations concern height, protected land and upper-floor side windows. [1]
An upper storey added above an earlier extension is not assessed only as the new upper-floor box. The total enlargement and the original rear wall remain relevant. The roof pitch should, so far as practicable, match the original house where the Class A condition applies. [1]
What does a 45-degree test actually show?
Brent's Residential Extensions & Alterations SPD uses a 45-degree line drawn in plan from the middle of the nearest neighbouring habitable-room window when assessing the position and depth of two-storey rear extensions. Its diagram and text define the local approach. That is a Brent planning-design test, not a rule for every borough. [2]
Ask your designer to identify the source, drawing direction, reference point and relevant window before presenting a diagram labelled “45 degrees”. A line drawn from the wrong window or on the wrong drawing cannot demonstrate compliance with the council's test.
Passing a diagram is not the whole assessment
The council's design guidance also considers the scale and relationship of the extension to the house and neighbouring properties. A line on a plan does not, by itself, resolve the appearance of a tall wall, the location of new windows or the roof form. Review plans, elevations and sections together. [2]
Conversely, a local design-guide projection is not a permitted development allowance. For example, Brent's guidance for planning proposals distinguishes attached and detached two-storey rear additions, while the national Class A multi-storey rear projection limit remains 3m. Keep those two assessments separate. [2] [1]
What information helps?
Ask for a measured record of neighbouring window positions, rear additions, levels and boundaries. Mark the existing and proposed outlines clearly so the council can assess the change. Where a particular light issue remains unresolved, ask the case officer what additional evidence would address it rather than commissioning a generic report with no defined question.
Structural feasibility also needs its own review. Existing walls and foundations may not be adequate for the new loads, especially where another storey is proposed over an earlier addition. [3]
Finally, planning and party wall procedures are separate. Agreement to party wall works does not replace the planning assessment, and a planning approval does not remove the need for applicable notices. [4]