A wraparound is not automatically prohibited; equally, it is not automatically permitted because each part looks small on its own. [1]
Why the combined width matters
Class A restricts an extension beyond a side wall to a single storey, no more than 4m in height and no wider than half the original house. Where an enlargement also projects beyond a rear wall, the rear limitations apply too. The government's technical guidance illustrates combined extensions which fail because the total enlargement is too wide. [1]
Draw the original house outline over the proposed plan. Mark each original side and rear wall, and test the connected enlargement from those reference points. Do not measure just the narrow strip beside the kitchen and ignore the rear element attached to it.
A stepped rear creates more than one test
An original rear wing can produce several relevant rear walls. The permissible projection is not necessarily measured only from the furthest-back wall. A scheme which looks modest beyond the wing can project too far beyond another original rear wall. [1]
The larger-home-extension prior-approval procedure can address the increased rear depth of an otherwise eligible proposal. It does not remove the side-width, height, site-coverage or protected-land restrictions. A successful neighbour discussion cannot override a failed dimensional test. [1] [2]
Building it in stages does not reset the rules
Where a new enlargement joins an existing extension, the combined enlargement is assessed against the relevant Class A limitations. That remains important whether the earlier work had planning permission or was permitted development. The technical guidance specifically addresses side and rear work undertaken in separate stages. [1]
Before paying for separate packages of drawings or building work, ask for the eventual completed arrangement to be checked. The planning position of stage one is not a certificate of lawfulness for stage two.
What a planning application should demonstrate
Where permitted development does not fit, the application should explain the proposed scale and relationship with the house and neighbours. Use the council's current design guide to identify the relevant assessment of projection, boundary height, light and appearance. Brent's extension SPD is one local example; another borough may use different tests. [3] [4]
Ask for a rear-only alternative to be retained during design development. Compare the extra usable space, roof arrangement and neighbour-facing mass of the wraparound against that alternative. This helps establish what the larger scheme achieves, rather than treating maximum site coverage as the brief.
Coordinate the technical design
The structural opening, foundations and existing drains should be investigated before the layout is finalised. An engineer may need information about concealed construction, and a sewer agreement may require revisions to the footprint. [5] [6]
Do not leave boundary details to the builder's interpretation. Party wall requirements are separate from planning, and the legal position of a new boundary wall differs from work to an existing party wall. [7]