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Porch Calculator — Foundations, Structure & PD Limits
Calculate porch material quantities and determine whether the porch is exempt from Building Regulations (under 30m² Schedule 2 Class 7) or requires a full application.
450mm is the frost-protection minimum. Use 700mm+ on shrinkable clay or near trees (NHBC Standards Ch 4.2). Concrete volume assumes a 0.45m wide strip - the practical minimum; 600mm is often used to lay bricks off the strip.
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How We Calculate This
This calculator determines porch material quantities and checks Building Regulations exemption status.
Exemption check
Porches added at ground level with a floor area of 30m² or less are exempt from Building Regulations under Schedule 2 Class 7 (Class 7 sets no height limit). Over 30m² triggers full compliance with Part L, B, P and structural requirements. (Permitted Development for planning is a stricter test: 3m² floor area, no higher than 3m, and at least 2m from a boundary fronting a highway — these are separate from the Building Regs test.)
Material quantities
Foundation concrete, brickwork and roof area are calculated from the porch dimensions, wall type and roof type.
Frequently Asked Questions
If the porch is added at ground level and has a floor area of 30m² or less, it is exempt from Building Regulations under Schedule 2 Class 7 (provided any glazing meets Part K). Class 7 sets no height limit — the 3m height limit is a separate Permitted Development (planning) test, not a Building Regs condition. Over 30m² floor area, full Building Regulations apply including Part L insulation, Part B fire safety and structural requirements. Adding controlled fittings such as fixed heating or new electrics can still bring those specific works under Building Regulations (e.g. Part P). Note: the Building Regs exemption (30m²) is separate from Permitted Development (3m²) — see the planning question below.
A porch is usually permitted development if: the floor area is 3m² or less, it is no higher than 3m above ground level, and no part is within 2m of a boundary fronting a highway. In conservation areas, Article 4 directions or on listed buildings, planning permission may be needed regardless of size.
A porch typically needs a strip foundation 450mm deep and 450mm wide around the perimeter. On clay soils or near trees, deeper foundations may be needed. For small porches under 3m², some local authorities accept a concrete pad or slab foundation without deep strip footings.
If the porch is Building Regs exempt (under 30m² Schedule 2 Class 7), no insulation is required by Building Regulations — but the original front door must be retained as the thermal-separation boundary (AD L Vol 1 2021, paras 0.14 and 10.12). If the porch is not exempt, Part L applies and new elements must meet AD L Vol 1 2021 Table 4.2 (new elements in existing dwellings): walls 0.18, floor 0.18, roof 0.15, windows 1.4, doors 1.4 W/m²K. If you remove the original front door, making the porch part of the house, all porch elements must be insulated to these Part L standards regardless of size.
As an indicative 2026 UK guide, a small brick porch (under 3m²) typically costs around £3,000–£6,000, and a larger porch (3–6m²) requiring Building Regulations around £5,000–£12,000 depending on specification. Full brick walls cost more than a dwarf wall with glazing. These are rule-of-thumb ranges only — costs vary significantly by region, access and finish; always get itemised quotes from local contractors.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.