Garden rooms & outbuildings

1. 1

   Check permitted development

   Confirm the building is single storey, under 2.5 m high within 2 m of a boundary, and not covering more than half the garden.

2. 2

   Choose and set out the base

   Pick a concrete slab, ground screws or a bearer and pad base to suit the ground, then set out square and level.

3. 3

   Build the base

   Dig out, compact a sub-base, and lay the slab or install screws so the finished platform is level within a few millimetres.

4. 4

   Erect the structure

   Build or assemble the frame, fix the wall panels, and get the roof watertight in the same visit if you can.

5. 5

   Insulate and line

   Insulate floor, walls and roof, add a vapour control layer and line out, because an uninsulated room is unusable half the year.

6. 6

   Run services

   Take a dedicated armoured supply from the consumer unit, buried at 450 mm or more with warning tape, installed and certified by a qualified electrician.

7. 7

   Finish outside

   Fit cladding, guttering and a soakaway or water butt for the roof run-off, and clear the ground around the base.

• Outbuildings are permitted development if they don't exceed 50 % of the curtilage, are single-storey, eaves ≤ 2.5 m, ridge ≤ 4 m (dual-pitch) / 3 m (other), and the building is not in front of the principal elevation. ^[1]
• Within 2 m of a boundary the maximum overall height drops to 2.5 m. ^[1]
• Buildings over 15 m² with sleeping accommodation, or over 30 m² regardless, trigger building regulations. ^[2]
• All fixed electrical installations must comply with BS 7671 and notifiable work needs Part P sign-off. ^[3][4]

• Before slab excavation or trenching the SWA supply, CAT & Genny sweep (HSE HSG47). The cable run from the consumer unit to the garden room almost always crosses incoming services. Full method in the Safety guide.
• Slab base (non-habitable): 100 mm C25 concrete on 100 mm MOT Type 1, A142 mesh on a 1200-gauge DPM. Pad foundations are an option on stable ground.
• Habitable garden room (year-round office, gym, studio): 150 mm C28/35 slab over 100 mm of rigid insulation on a continuous DPM, with A193 mesh. Building Regs Part L + Part C apply, talk to building control before you pour.
• Run a dedicated SWA armoured cable from the consumer unit on a 32 A RCBO for a typical office; 6 mm² for runs under ~30 m.
• Insulate to a U-value of 0.18–0.22 W/m²K in walls and 0.15 in the roof for a year-round usable room.
• Site the room 1 m from any boundary if you can, easier rendering, gutter access, and dispute avoidance.

The base decides how long the building lasts and how much the groundworks cost. Three options cover almost every UK garden.

• Concrete slab: 100 mm C25 on 100 mm compacted MOT Type 1 with A142 mesh over a 1200-gauge DPM for a garden store or non-habitable room. It is the most tolerant base and the easiest to build square, but it is the most disruptive to remove later.
• Insulated slab for year-round rooms: 150 mm C28/35 with A193 mesh over 100 mm of rigid insulation on a continuous DPM. This is what stops an office feeling cold underfoot in January and is expected once Part L applies.
• Screw piles or ground screws: driven in a day, no spoil, no concrete, and ideal on sloping or awkward-access plots. Typically £30 to £60 a screw plus installation, with a treated timber frame spanning between them.
• Pad foundations with a suspended timber deck: a middle ground, cheaper than a full slab and good on stable ground, but needs careful setting out and a ventilated void beneath.
• On clay, expect deeper pads or piles because seasonal shrink and swell will move a shallow slab. Within influencing distance of a mature tree, take advice before pouring anything.
• Whatever the base, keep the finished floor at least 150 mm above surrounding ground and fall the ground away from the building.

• Assuming permitted development covers it. Height within 2 m of a boundary, verandas and balconies, and anything forward of the principal elevation all catch people out. A lawful development certificate costs a fraction of an enforcement notice.
• Treating it as a shed with a plug socket. A room used all year needs insulation, a vapour control layer, ventilation and a proper electrical supply, or it grows condensation and mould in the first winter.
• Running a garden supply on an extension lead. Outdoor supplies are SWA armoured cable on their own protected circuit, installed and certified by a registered electrician.
• No ventilation strategy. An airtight insulated box with a laptop and a person in it hits high humidity fast. Trickle vents plus a small extract fan, or a through-wall MVHR unit, solve it.
• Guttering that discharges straight onto the lawn. Take roof water to a soakaway at least 5 m from any building, or into a butt with an overflow.
• Forgetting the route in. A 3.6 m wall panel has to get past the side gate. Measure the access before the design is signed off.

• Basic uninsulated garden store or summerhouse, supplied and erected: roughly £2,000 to £6,000.
• Insulated garden office, fully finished and wired, 3 x 4 m: typically £15,000 to £30,000 depending on specification and glazing.
• Base alone: £1,500 to £4,000 for a slab including excavation and spoil removal, less for screw piles on easy ground.
• Electrical supply from the house consumer unit: £700 to £1,800 including cable, trenching and certification.
• Building regulations application where it applies: a few hundred pounds in fees, plus the cost of building to the standard, which is mainly insulation and glazing.