One of the most frequently asked questions at the start of a building project is how long the work will take. Understanding the realistic timescale for an extension project helps you plan your life around the disruption, make appropriate arrangements for living in or out of the property, and assess whether a contractor is being realistic about their programme.
Timescales for house extensions in London vary significantly depending on the type and scale of the extension, the complexity of the structural work, the specification of the finishes, and the condition of the existing property. The figures below represent realistic ranges for typical projects and assume a well-planned, competently managed build.
A small single-storey rear extension of around three to five metres with a flat roof, a straightforward structural opening, and standard finishes typically takes eight to twelve weeks on site. This assumes foundations are straightforward, no sewer build-over is required, and the structural openings into the existing house do not involve complex temporary support.
A larger single-storey rear extension of six to eight metres or a side return extension typically takes ten to sixteen weeks on site. The additional complexity of a larger structure, more extensive drainage work, and the greater volume of finishing work extends the programme.
A double-storey extension is significantly more complex than a single-storey project and typically takes sixteen to twenty-four weeks on site. The additional structural work, scaffolding, roofing to the new first floor level, and the greater number of rooms to finish all extend the programme substantially.
A wrap around extension typically takes fourteen to twenty weeks depending on its size and the complexity of the corner structural junction.
The pre-construction phase, which includes obtaining planning permission, preparing structural drawings, and discharging any planning conditions, typically adds three to six months to the overall timeline before work starts on site. Planning applications for householder extensions take eight weeks from validation to decision in most London boroughs. Building regulations applications can run in parallel with construction if a building notice route is used.
Contractors who quote timescales significantly shorter than these ranges are either very well-resourced with large teams or they are being unrealistic. A programme that is too compressed typically leads to poor quality work, health and safety risks, or a contractor who disappears from site for extended periods to keep other projects moving.
RCB Design and Build provides realistic project programmes at the pre-construction stage, based on the actual scope of work. We keep clients informed of progress against programme throughout the build.