Planning a garden office in Sussex: permitted development, insulation, the lot
Published 2026-03-25 · by Joshua Lewis
Most garden offices in Sussex fall under permitted development — no planning application needed — as long as the building stays under 2.5 metres tall (if within 2m of the boundary), covers less than 50% of your garden, and is used for purposes incidental to the house. Get the detail wrong and you'll need retrospective planning.
Permitted development — the rules that matter
- Must be to the rear of the house (or to the side, in some cases)
- Not more than 2.5m high if within 2m of any boundary
- Not more than 4m high pitched or 3m high flat if further than 2m from boundaries
- No more than 50% of the garden covered by outbuildings
- Must not be used as a separate dwelling (no self-contained living)
- In conservation areas, National Parks (South Downs) or listed buildings: additional restrictions
Build-up we use
Typical spec for a Lewis Brothers garden office:
- Concrete or screw-pile foundation
- Insulated floor (100mm PIR, chipboard deck)
- Timber frame walls, 100mm PIR between studs, breather membrane, cladding battens, cladding
- Insulated warm-roof build-up
- Double-glazed doors and windows
- First-fix electrics run back to consumer unit (qualified sparks)
- Internal finish: plasterboard, skimmed, decorated
End result: somewhere you can work year-round without a space heater.
Rough costs in 2026
Bespoke on-site build for a 4m × 3m insulated office: £14,000–£22,000 depending on cladding choice, glazing, and internal fit-out. Cheaper than any pre-fab equivalent at the same spec, and you get exactly the layout you want.
Common questions
Can I run plumbing and a toilet?
Yes but check permitted development — plumbed facilities can push you out of permitted scope in some cases. We can advise.
Will it need building regs?
Not usually if it stays under 15m² floor area and away from boundaries. Over 15m² or close to boundaries, building regs apply. We'll flag it up front.