Extensions & Conversions Calculators
Extension costs, foundations, roofing, garage conversions and building project calculators.
Extension Cost Estimator
Estimate extension cost per m² for single and two storey with material breakdown.
Extension Foundation Calculator
Calculate extension foundation depth and width with tree proximity per NHBC Table 4.2.
Extension Roof Calculator
Calculate extension roof materials for pitched or flat with insulation.
Extension Wall Calculator
Calculate extension wall materials: blocks, insulation, bricks, DPC and wall ties.
Garage Conversion Calculator
Calculate garage conversion insulation, replacement door structure and services.
Extension Glazing Calculator
Calculate extension glazing area against Part L 25% maximum with U-value check.
Extension Heating Calculator
Calculate extension heating from room heat loss with radiator or UFH sizing.
Extension Electrical Calculator
Calculate extension electrical circuits, consumer unit ways and SWA for detached.
Extension Drainage Calculator
Calculate extension drainage connection, inspection chambers and build-over requirements.
Side Return Extension Calculator
Calculate side return extension steelwork, glazing and drainage re-route.
Extension Steel Beam Calculator
Calculate extension opening steel beam from tributary area loading.
Extension PD Checker
Check extension dimensions against permitted development depth, height and eaves limits.
Conservatory Base Calculator
Calculate conservatory base foundation, slab, DPM and dwarf wall.
Conservatory Roof Replacement Calculator
Calculate conservatory solid roof replacement structural upgrade and insulation.
Porch Calculator
Calculate porch materials with PD check for under/over 3m².
Orangery Calculator
Calculate orangery brick columns, lantern roof, glazing and structural beam.
Extension Party Wall Calculator
Determine extension party wall notice requirements from boundary distance.
Annexe Calculator
Calculate annexe self-contained requirements with utility connections.
Extension Slab Calculator
Calculate extension ground floor slab buildup: hardcore, DPM, insulation and concrete.
Extension Ventilation Calculator
Calculate extension Part F ventilation with trickle vents and extract in wet rooms.
Extension Fire Safety Calculator
Calculate two-storey extension fire safety: protected stairway, FD30 doors and alarms.
Flat Roof Extension Calculator
Calculate flat roof extension buildup with insulation, membrane, upstand and drainage.
22 free calculators in Extensions & Conversions
Extensions & Conversions Calculators for UK Builders
Extensions and conversions remain the most popular way for UK homeowners to add living space without moving. Our calculators help you estimate costs for single and double storey rear extensions, side return extensions, loft conversions, garage conversions, and orangeries. From foundation costs and steelwork to roofing and finishing, these tools provide detailed breakdowns that help you quote accurately and manage client expectations on budget and timescale.
In England and Wales, many extensions fall under permitted development rights as defined by the Town and Country Planning (General Permitted Development) Order 2015 (as amended), though size limits, boundary distances, and height restrictions apply. Larger projects require full planning permission. All extensions must comply with Building Regulations, including Approved Document L (conservation of fuel and power) for thermal performance, Part E for sound insulation between dwellings, and Part M for accessibility. Structural calculations to Eurocodes are required for steelwork and foundation design.
These calculators factor in regional cost variations across the UK, from London and the South East to Scotland and Northern Ireland. They cover preliminary costs such as party wall surveyor fees, architectural drawings, and Building Control application fees, alongside construction costs for groundworks, masonry, joinery, and M&E services. Use them to produce professional, itemised quotations that give your clients confidence in both your pricing and your expertise.