Garage Conversion Calculator — Insulation & Compliance

Most garage conversions do not need planning permission as they are internal alterations. However, you will need a Building Regulations application as the space must meet standards for structure, insulation, ventilation, fire safety and drainage. If the conversion materially changes the external appearance (e.g. new windows) or the property is in a conservation area, planning permission may be required.

A garage conversion creates a new thermal element, so the floor must achieve a U-value of 0.18 W/m²K per Approved Document L Volume 1 (2021 edition), Table 4.2. The most common approach is to overlay the existing slab with a DPM, PIR insulation (e.g. Celotex/Kingspan, λ≈0.022 W/mK) and 18mm T&G chipboard. To meet 0.18 W/m²K you typically need around 100–120mm of PIR — confirm the exact thickness with a BS EN ISO 13370 perimeter-to-area calculation, as a favourable perimeter/area ratio can reduce it. This raises the floor level by roughly 120–140mm, so check door thresholds and step heights. If headroom is limited, higher-performance insulation (e.g. vacuum-insulated panels) or excavating the slab may be needed.

As an indicative 2026 guide, a single integral or attached garage conversion typically costs around £12,000–£24,000 (national average roughly £16,800), and a detached or double garage £18,000–£30,000+. These figures vary widely by region, specification and whether plumbing/an en-suite is included — London and the South East commonly add 15–25%. Always obtain itemised local quotes; the ranges here are illustrative, not fixed prices. A conversion is usually the most cost-effective way to add habitable space to a property.

The garage door is removed and the opening is bricked up, usually with a window and/or entrance door. The infill must include DPC, cavity insulation and proper lintels. The brickwork should match the existing house as closely as possible. Full-width glazing is an alternative but requires structural assessment.

A well-executed garage conversion is widely reported to add in the region of 10–15% to a property value (potentially up to around 20% where it creates an extra bedroom or en-suite), often exceeding the cost of the work — though this is an indicative market estimate, not a guaranteed figure. Losing a garage can reduce value in areas where off-street parking is at a premium. Check local market conditions and consider retaining a parking space on the former garage drive.