Permitted development or planning permission
Many single-storey rear extensions fall within permitted development, subject to depth, height, materials and the position of the property — and permitted development rights are frequently removed in conservation areas or by an Article 4 direction. A lawful development certificate is worth obtaining even where consent is not needed, because it is what a purchaser’s solicitor will ask for. Larger or two-storey proposals, and anything on a listed building, need full permission.
Which extension suits the house
A rear extension adds depth and suits houses with garden to spare. A side-return extension captures the alley beside the original outrigger and transforms the plan of a Victorian terrace without losing much garden. An outrigger extension builds up over the existing back addition, adding bedrooms rather than living space. A wrap-around does both and is the most structurally involved. The right answer is usually determined by daylight to the existing rooms and by where the staircase lands, not by floor area alone.
Structure and the party wall
Opening up the rear of a house means beams, and beams mean bearings on the party walls: notifiable under the Party Wall Act, so notices and awards run in parallel with design. Foundations next to a neighbouring structure may need to be deepened, and any tree in the vicinity affects foundation depth on London clay. These decisions are made by the engineer, but they are what determines whether the groundworks take three weeks or seven.
Glazing, rooflights and the junction
Glazing is the element clients care most about and the one most often ordered too early. Architectural aluminium and steel systems should be surveyed from the completed structural opening, not from the drawing, and lead times of eight to twelve weeks are normal. Large rooflights may need a crane and therefore a lift plan, road permissions and an exclusion zone — at Ceylon Road in Brook Green the rooflights were craned into position over the completed structure in a single possession.
Programme and living through it
A straightforward single-storey rear extension typically runs three to five months; a side-return with a kitchen and external works, four to seven; a two-storey outrigger with a loft conversion, considerably longer. The kitchen is usually the casualty: plan the temporary kitchen and the route to the garden before work starts, because both will be needed for longer than anyone expects.