A single storey extension is an addition of one storey to the ground floor, most often at the rear or into a side return. It is the commonest building project in the country and the one with the most predictable failure mode: too much depth, not enough thought about where light reaches once the new roof is on.
What is a single storey extension, in the terms the rules use
One storey, at ground level, attached to the house, with its own roof. The rules treat it by its depth from the original rear wall, its height, its position relative to boundaries and whether it sits beyond a side elevation. What counts as the original wall is the house as first built or as it stood at a fixed historic date, not as you bought it, which is why a previous owner's addition can quietly consume the allowance you were counting on.
Depth has diminishing returns
The early metres convert a cramped kitchen into a working one. Later metres buy a deeper plan whose centre is further from any window, so the room gets bigger and gloomier at the same time. Rooflights over the old rear wall line, a wider opening into the original room, or going wide instead of deep are the three fixes, and all three are free at sketch stage.
A small single storey extension can be the better project
A small extension that squares off an awkward corner, widens a galley or adds a utility often changes daily life more than a large one, and it does so without a long programme, a large party wall exercise or half the garden. On a tight plot the calculation is simple: which square metres are currently unusable, and what would it take to make them usable.
What has to be proved either way
Size does not reduce the technical obligations. Foundations have to suit the ground, and near trees on shrinkable clay that means depth. The wall, floor and roof build ups have to meet the insulation standard. Ventilation and, where a habitable room is involved, escape have to work. A structural opening between old and new needs a designed beam and a bearing. A very small extension meets every one of these, just fewer times.
A steel beam extension opening, and the calculations that come first
The wall between the old house and the new room is usually load bearing, and what replaces it is a steel beam sized by an engineer for the loads above it. That calculation comes before the quote rather than after, because the beam's depth decides the ceiling line, its bearings decide whether padstones or a new pier are needed at each end, and its weight decides how it gets into the house and whether a crane is involved. A builder who has priced the opening without a calculation has priced a guess, and the honest ones say so. Ask for the engineer's calculations and the beam schedule as part of the tender pack, so every builder is pricing the same steel.
Questions people ask about single storey house extension
Does a single storey extension always need planning permission?
No. Many are permitted development, but the allowance depends on depth, height, boundaries and what has been built before, and it does not apply to flats.
What is the original house?
The dwelling as first built, or as it stood at the historic date the rules use. Earlier extensions by any owner count against the allowance.
Is building control needed even for a small one?
Yes. Building regulations apply to the construction whatever the planning position, and the approval is separate.
Why do foundations sometimes cost more than expected?
Because ground conditions are only known once a hole is dug. Clay, made ground, a nearby tree or an old drain all push the depth down.