Do I need planning permission for a garden room?

Most garden rooms fall under permitted development rights and do not need planning permission, provided: they are single-storey with a maximum eaves height of 2.5m; they are not forward of the principal elevation; they cover no more than 50% of the garden; and they are not in a conservation area, AONB, or listed building curtilage. For buildings over 15 m² with sleeping accommodation, Building Regulations approval is required.

What insulation should I use for a garden room?

PIR boards (such as Celotex or Kingspan) are the most popular choice as they offer excellent thermal performance in a slim profile. 100mm PIR gives a U-value of approximately 0.22 W/m²K, meeting Building Regulations Part L. Mineral wool is a cheaper alternative but requires a thicker build-up for the same performance.

What U-value does a garden room need?

If your garden room requires Building Regulations approval (over 15 m² with sleeping accommodation, or if it has its own heating system), walls should achieve U-values of 0.28 W/m²K or better, roofs 0.18 W/m²K, and floors 0.22 W/m²K per Approved Document L. Many self-builders aim for better than the minimum for comfort.

What is the best roof covering for a garden room?

EPDM rubber membrane is the most popular choice for flat-roof garden rooms. It comes in a single sheet (no joins), has a 50+ year lifespan, and is easy to install. GRP fibreglass is also excellent and creates a seamless, hard-wearing finish but is more expensive and harder to DIY. Both are suitable for flat roofs with a minimum 1:60 fall.

How thick should the floor be in a garden room?

A typical garden room floor build-up is: concrete pad footings or screw piles, 150x50mm joists at 400mm centres, 100mm PIR insulation between joists, 18mm OSB or plywood deck, then your chosen floor finish. The total floor build-up is typically 200-250mm. A DPM (damp-proof membrane) should be placed beneath the floor to prevent moisture ingress.

Garden Room Calculator — Self-Build Materials Estimator

Estimate timber studs, insulation, plasterboard, cladding, roofing and all materials for a garden room or home office build.

Room Length (External)

External length of the garden room

Room Width (External)

External width of the garden room

Room Height

Internal floor-to-ceiling height (max 2.5m for PD)

Insulation Type

PIR offers best performance for thickness

External Cladding

Timber cladding is most popular for garden rooms

Roof Covering

EPDM is the most common flat roof covering

Number of Windows

Assumed 1200x1000mm each

Number of Doors

Assumed 900x2100mm each

Wastage Allowance

Extra material for cutting and waste

How We Calculate This

This calculator estimates the key materials needed to build a timber-frame garden room or home office to a standard specification. It uses common UK construction practices and standard material sizes.

Standard specification

Wall frame: 100x50mm CLS studs at 600mm centres
Insulation: 100mm PIR (U-value ~0.22 W/m²K) or mineral wool
Sheathing: 18mm OSB on external face
Internal lining: 12.5mm plasterboard
Floor: 150x50mm joists at 400mm centres on pad footings
Roof: Flat roof with EPDM or GRP membrane

What is included

Timber studs, plates, floor joists and roof joists
Insulation for walls, roof and floor
Plasterboard (internal) and OSB (sheathing)
External cladding area
Roofing membrane, DPM and vapour barrier

Window and door openings are deducted from wall areas. Standard window size assumed as 1200x1000mm and door as 900x2100mm. Wastage is applied to account for cutting and fitting.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.