What is the largest building you can build without planning permission?

Permitted development doesn't set a single maximum floor area for outbuildings — the practical cap is the 50% rule. On a large plot you could lawfully build a very substantial garden building; on a small terrace garden, the same rules might allow almost nothing.

Height is where most projects hit the ceiling: 4 metres overall for a dual-pitched roof, 3 metres for any other roof shape, and just 2.5 metres if any part of the building sits within 2 metres of a boundary — which is why boundary-line garden offices are almost always flat-roofed at 2.5 metres.

Outbuildings must be single storey, with eaves no higher than 2.5 metres, no verandas or raised platforms, and they can't sit forward of the front of the house. Crucially, they must be for purposes "incidental" to the house — a gym, office or store, not a self-contained annexe or rental unit, which need permission.

Building regulations run on a separate track: garden buildings under 15 square metres with no sleeping accommodation are generally exempt; between 15 and 30 square metres they're exempt only if at least a metre from a boundary or built substantially of non-combustible material; above 30 square metres, approval is required.

On designated land the rules tighten again — in conservation areas, for example, outbuildings at the side of a house need permission. If your "biggest possible building" plan is anywhere near these limits, confirm the constraints on your specific plot before ordering the steel frame.