What is the formula for hip to gable roof volume?

The added volume created by extending a hip roof end to a vertical gable is calculated using the triangular pyramid volume formula: Volume = (1/6) × Span × Depth × Height. For a symmetric roof where the depth equals half the span, this simplifies to Volume = (1/12) × Span² × Height.

What are the UK Permitted Development volume allowances for lofts?

Under UK Permitted Development guidelines, the maximum additional roof space allowed without planning permission is 40 cubic metres (approx. 1,412 cu ft) for terraced houses, and 50 cubic metres (approx. 1,765 cu ft) for semi-detached and detached homes.

Does a hip to gable conversion include rear dormer volume?

No. The hip-to-gable calculation only covers the volume added by turning the side sloped hip into an upright gable wall. If you also build a rear flat-roof box dormer, its volume must be calculated separately and added to the total for Permitted Development limits.

How is the gable wall vertical surface area calculated?

The gable wall added is a vertical triangle calculated as: Area = 0.5 × Base Span × Ridge Height. This metric is essential when ordering exterior cladding, tiles, or facing bricks.

🏠 Loft Conversion & Permitted Development Estimator

Hip to Gable Roof Volume Calculator

Compute the exact cubic capacity, new vertical gable wall area, and planning permission limits (40m³ & 50m³ PD allowances).

Measurement System:

Gable Base Span (m) Wall-to-wall roof width

Hip Run / Depth (m) Ridge apex to hip eaves

Ridge Vertical Rise (m) Ceiling floor to ridge apex

Conversion Side(s) Number of hip ends converted

Quick Presets:

Permitted Development (PD) Volume Tracker

UK planning laws grant typical allowances of 40 m³ for terraced houses and 50 m³ for semi-detached / detached houses without requiring full planning permission.

Terraced Allowance (40 m³ / 1,412 ft³) 0%

Semi / Detached Allowance (50 m³ / 1,765 ft³) 0%

About Hip to Gable Roof Volume Calculations

A hip roof slopes back towards the ridge on its ends. In a hip-to-gable conversion, the sloping triangular roof face (the hip) is dismantled and replaced with an upright vertical wall extending straight up to the ridge apex, carrying the horizontal roof ridge across to meet it.

1. Added Volume per Hip = (1/6) × Span × Depth × Rise

2. Symmetric Hip Simplification (Depth = Span / 2) = (1/12) × Span² × Rise

3. New Gable Wall Vertical Surface Area = 0.5 × Span × Rise

4. Total Conversion Volume = Single Hip Volume × Number of Hips Converted

Note: If you are planning a combination hip-to-gable and rear box dormer conversion, the volume calculated here only accounts for the triangular hip portion. You must sum both calculations to verify total compliance with Permitted Development caps.

Frequently Asked Questions

Loft conversions, hip roofs, planning rules, and building maths

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