Garden Room Calculator — Self-Build Materials Estimator

Most garden rooms fall under permitted development (Class E outbuildings) and do not need planning permission, provided: they are single-storey with a maximum eaves height of 2.5m (and overall height of 3m for a flat or single-pitch roof, or 4m for a dual-pitch roof — reduced to 2.5m overall if within 2m of any boundary); they are not forward of the principal elevation; together with other outbuildings they cover no more than 50% of the land around the original house; and the property is not in a conservation area, AONB, or the curtilage of a listed building (where extra restrictions apply). Building Regulations are separate from planning — see the size and sleeping-accommodation rules below.

PIR boards (such as Celotex or Kingspan) are the most popular choice as they offer excellent thermal performance in a slim profile. Note that 100mm PIR (λ ≈ 0.022 W/mK) has a board-alone U-value of about 0.22 W/m²K, but once the timber studs, sheathing, plasterboard and surface resistances are included the installed wall typically computes to roughly 0.27–0.30 W/m²K (timber studs bridge about 15% of the wall). To meet the Part L extension wall limit of 0.18 W/m²K you usually need ~100mm PIR between studs plus a continuous insulated layer over them (e.g. 25–50mm PIR sheathing board), or thicker PIR. Mineral wool is a cheaper alternative but needs a thicker build-up for the same performance.

A garden room only needs to meet Approved Document L if it requires Building Regulations approval (broadly: over 30 m², or with sleeping accommodation, or fitted with its own fixed heating). Under AD L Volume 1 (2021 edition, incorporating 2023 amendments) the limiting U-values are: if the build is treated as new fabric in/extending an existing dwelling (Table 4.2) — wall 0.18, roof 0.15, floor 0.18, windows 1.4 W/m²K (or Window Energy Rating Band B); if assessed as a stand-alone new building (Table 4.1) — wall 0.26, roof 0.16, floor 0.18, windows 1.6 W/m²K. Note the Table 4.2 roof figure (0.15) is a single value covering all roof types — there is no separate flat-roof/pitched-roof split in that table. Many self-builders better these minimums for year-round comfort. The older 0.28/0.18/0.22 figures come from the withdrawn 2010/2013 edition and no longer apply.

EPDM rubber membrane is the most popular choice for flat-roof garden rooms. It comes in a single sheet (no joins), has a BBA-certified 50+ year life expectancy, and is straightforward to install. GRP fibreglass is also excellent and creates a seamless, hard-wearing finish but is more expensive and harder to DIY. Whichever you choose, BS 6229:2025 (the current flat-roof code of practice, which replaced BS 6229:2018 on 31 December 2025) requires a minimum completed fall of 1:80; for a small roof like a garden room (under 50 m²) it recommends aiming for a design fall of about 1:60 as a sensible starting point, so the minimum 1:80 still remains after deck deflection and construction tolerance. Designing flat or to a token fall risks ponding.

A typical garden room floor build-up is: concrete pad footings or screw piles, 150x50mm (47x145mm CLS) 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. Note 47x145 C16 joists at 400mm centres span only about 2.97m at a 1.5 kN/m² domestic floor load (C24 reaches about 3.27m), so for wider rooms use deeper joists, the higher C24 grade or an intermediate beam (BS 5268-7.1 / TRADA span tables). A DPM (damp-proof membrane) should be placed beneath the floor to prevent moisture ingress.