Start with the rooms and activities you need to accommodate, then compare layouts against the space, budget and neighbouring properties. [1]
Terraced houses: examine the rear and the side return
For a terrace, compare a straight rear addition with infilling beside an existing rear wing. Draw the rooms that remain inside the original house as carefully as the new room: an attractive garden-facing kitchen is not the only measure of whether the layout works. Historic England's guidance highlights the relationship between the main house, rear additions and the remaining yard or garden. [2]
Do not label an infill proposal “rear-only” without checking the original walls. Where an extension projects beyond both a side wall and a rear wall, both sets of permitted development limits can apply. A previous owner's additions also affect the calculation. [3]
Semi-detached houses: consider the pair
Compare a rear extension, a side extension and a two-storey option against the appearance of both halves of the building. Ask the designer to show the street elevation, not simply your individual floor plan. Brent's adopted extension guidance, for example, addresses the scale and relationship of additions to the original dwelling and adjoining homes; its local design tests must not be treated as universal London allowances. [4]
Also compare what happens to side access. Put refuse storage, garden access and a practical construction route on the options drawings rather than treating the space beside the house as automatically available for rooms.
Detached houses: more options, not unlimited rights
Detached status changes certain rear-extension depth limits, but it does not remove restrictions on height, site coverage, side extensions or protected areas. The ordinary single-storey rear permitted development limit is 4m for a detached house, compared with 3m for other houses; the larger-extension route has separate prior-approval requirements. These figures are not planning promises. [5]
A detached house can still have an excavation close enough to a neighbouring building to trigger party wall procedures. The relevant test is the work and its relationship to the neighbouring structure, not whether the two houses touch. [6]
Compare like-for-like options
Ask for a comparison that records:
- the usable rooms gained, furniture positions and the garden retained;
- the planning route and the neighbour-facing elevations;
- structural openings, foundations and known drainage constraints;
- the complete scope to be priced, including alterations inside the existing house.
Structural feasibility should be checked before committing to a wide opening or another floor. An engineer may need investigations to establish existing foundations and load paths. A public sewer can also require a separate agreement and a change to the proposed footprint. [7] [8]
Keep the smallest workable option in the comparison. It provides a useful test: is the extra area in a larger scheme solving a genuine need, or simply increasing the amount of construction?