Building an Extension Calculators
A house extension is a major project involving every construction trade, from groundworks to decoration, and the order the trades arrive in matters as much as the quantities they need. This page is a working template for the whole job: the task sequence from permissions to paint, what you will be ordering at each stage, where the Building Regulations touch the work, and the calculator that puts numbers on each step.
Two decisions shape everything else. First, the paperwork: planning or permitted development, building regulations approval, and any Party Wall Act notices all need to be settled before work starts, because they can change the design. Second, the structural design: an engineer's calculations for foundations, lintels and steels are needed early, and everything from the concrete order to the brick count follows from them.
Work in this order
Prove the project before anyone digs
Check whether the design falls within permitted development with the Extension Permitted Development Calculator, and what a planning application costs with the Planning Fee Calculator if you need one. If the work is on or near a shared boundary, work out which Party Wall Act notices apply with the Extension Party Wall Calculator. Set the budget with the Extension Cost Calculator and price the building control application with the Building Control Fee Calculator. Structural elements (beams, foundations) need a structural engineer's design before work starts.
Groundworks, foundations and below-ground drainage
Size the dig and the muck-away with the Groundwork Excavation Calculator, then the trench and concrete with the Extension Foundation Calculator. Below-ground drainage runs are planned now and laid while the ground is open: the Extension Drainage Calculator covers pipe sizing, gradients and inspection chambers. Concrete needs time to cure before anything is built off it.
Ground floor slab
The floor is a layered build-up: hardcore, sand blinding, damp-proof membrane, insulation, then concrete. The Extension Slab Calculator quantifies the whole stack, with the Type 1 Sub-Base Calculator for the hardcore layer.
The masonry shell
Cavity walls go up next: inner block leaf, cavity insulation, outer brick skin, wall ties, and a damp-proof course kept above finished ground level. The Extension Wall Calculator covers the full wall build-up, with the Lintel Sizing Calculator for openings and the Extension Steel Calculator plus Padstone Sizing Calculator where the engineer has specified steel.
Roof on, rainwater connected
Get the roof structure, covering, flashings and guttering done before starting inside. Water getting past an unfinished roof soaks insulation and masonry, so the shell must be weathertight before internal trades begin. Start from the Extension Roof Calculator for pitched or flat coverings, the Lead Flashing Calculator for the abutment where the extension meets the house, and the Gutter Sizing Calculator for the rainwater goods.
Windows and external doors
Glazing closes the envelope, and Part L limits how much of the extension can be glass. Check the area and U-value position with the Extension Glazing Calculator before ordering.
First fix: electrics and plumbing
Cables and pipes go in while the walls and ceilings are still open. Once the boards are up, routing services is much harder, so first fix always comes before plasterboard. Plan circuits with the Extension Electrical Calculator and heating with the Extension Heating Calculator.
Insulate, board, plaster
Insulation sits across several stages (under the slab, in the cavity, in the roof), so check the whole fabric against Part L with the Part L Compliance Calculator and the U-Value Calculator. Then plasterboard with the Plasterboard Calculator, and skim once the boards are up with the Skim Coat Calculator.
Screed, second fix and decorating
Floor screed goes down with the Screed Calculator and needs its drying time before final floor finishes. Second fix brings sockets, switches, radiators, skirting and doors, and decoration comes last with the Paint Coverage Calculator.
What you will be ordering
The groundworks stage is concrete, hardcore and a damp-proof membrane, plus reinforcement where the engineer specifies it. The shell is facing bricks, blocks, mortar (cement and sand), wall ties, damp-proof course, cavity insulation, cavity closers and lintels. The roof adds structural timber, felt or membrane, battens, tiles or a flat-roof covering, flashings, fascia, soffit and guttering.
Inside, first fix consumes cable, back boxes, pipe and fittings; then come insulation boards, plasterboard with its screws, tape and jointing compound, finish plaster, screed, floor finishes, skirting and paint. Order stage by stage rather than all at once: quantities firm up as the build progresses, and materials stored on site for months get damaged or wet. Each calculator linked above returns the order quantities for its own stage.
Where the Building Regulations touch an extension
Approval itself
An extension needs a building regulations application whether or not it needs planning permission. Estimate the fees with the Building Control Fee Calculator, and check the planning position with the Extension PD Calculator or the general Permitted Development Checker.
Structure
Beams, lintels, padstones and foundations must be designed or checked by a structural engineer; Building Control will expect the calculations. Our Lintel Sizing, Extension Steel and Padstone Sizing calculators explain each element and help you budget the structural stage.
Fire safety (Part B)
Two-storey extensions can change the escape-route picture for the whole house: the Extension Fire Safety Calculator covers protected stairways, fire doorsets, smoke alarms and escape windows.
Ventilation (Part F)
New habitable rooms and wet rooms have ventilation requirements: trickle vents, extract rates and purge ventilation are covered by the Extension Ventilation Calculator.
Drainage (Part H)
Below-ground drainage, gradients and soakaways are governed by Part H: see the Extension Drainage Calculator, Drainage Fall Calculator and Soakaway Sizing Calculator.
Energy (Part L)
Walls, floor, roof and glazing all carry U-value targets, and glazed area is capped relative to floor area: check with the Part L Compliance Calculator and the Extension Glazing Calculator.
Electrics (Part P)
New circuits in the extension are notifiable electrical work: the Part P Notification Calculator explains the self-certification and building control routes.
Party Wall Act
Work on a shared wall, or excavation near a neighbour's foundations, triggers notice requirements: use the Extension Party Wall Calculator or the Party Wall Checker.
Every calculator, by build stage
Planning, Costs and Permissions
Extension Cost Calculator
Estimate the whole build cost, from foundations to finishing, with regional pricing.
Extension Permitted Development Calculator
Check rear depth, eaves and ridge limits against permitted development rights.
Planning Fee Calculator
The application fee if the extension needs planning permission.
Extension Party Wall Calculator
Which Party Wall Act notices apply, based on boundary distance and excavation depth.
Building Control Fee Calculator
Estimate building regulations application and inspection fees.
Scaffolding Calculator
Scaffolding requirements and hire costs for the build.
Skip Size Guide
Choose the right skip size for construction waste.
Van Payload Calculator
Check your van can carry the material weights involved.
Groundworks, Foundations and Drainage
Groundwork Excavation Calculator
Dig volume, soil swell and lorry loads for the excavation.
Extension Foundation Calculator
Trench depth, width and concrete volume by soil type and tree proximity.
Foundation Calculator
Foundation depth, width and concrete volume for strip or trench fill.
Concrete Volume Calculator
Concrete volume for foundations, oversite slab and paths.
Concrete Mix Calculator
Cement, sand and aggregate quantities for site-mixed concrete.
Rebar Calculator
Reinforcement bar quantities for reinforced foundations.
Type 1 Sub-Base Calculator
Sub-base material for oversite preparation.
Extension Drainage Calculator
Below-ground pipe sizing, gradients, inspection chambers and build-over checks.
Drainage Fall Calculator
Correct gradient for foul and surface water drains.
Soakaway Sizing Calculator
Size a soakaway for the extension roof drainage.
Slab, Walls and Structural Openings
Extension Slab Calculator
The full ground floor build-up: hardcore, blinding, DPM, insulation, concrete and mesh.
Extension Wall Calculator
Cavity wall materials: blocks, bricks, insulation, wall ties, DPC and cavity closers.
Brick Calculator
Facing bricks for the outer leaf of cavity walls.
Block Calculator
Concrete blocks for the inner leaf and below-ground work.
Mortar Calculator
Sand and cement for bricklaying and blockwork mortar.
DPC Calculator
Damp-proof course lengths, cavity trays and stepped DPC quantities.
Lintel Sizing Calculator
Correct lintel sizes for window and door openings.
Padstone Sizing Calculator
Padstone dimensions for steel beam bearings.
Extension Steel Calculator
Indicative steel beam sizes for openings. Engineer design essential.
Roofing
Roof Area Calculator
Calculate actual roof area from plan dimensions and pitch.
Extension Roof Calculator
Tiles, felt, battens, membrane and insulation for pitched or flat extension roofs.
Roof Tile Calculator
Tile quantities for pitched roof extensions.
Flat Roof Membrane Calculator
Membrane and insulation for flat roof extensions.
Felt and Batten Calculator
Roofing felt and batten quantities.
Ridge Tile Calculator
Ridge tiles and mortar for the roof ridge.
Lead Flashing Calculator
Lead flashing for abutments where the extension meets the house.
Soffit and Fascia Calculator
Soffit boards and fascia lengths.
Gutter Sizing Calculator
Gutters and downpipes for the extension roof.
Windows, Doors and Glazing
Extension Glazing Calculator
Check glazed area against Part L limits and U-value compensation options.
Bi-Fold Door Calculator
Panel configurations, costs and U-values for bi-fold doors.
French Door Calculator
French door costs and Part L U-value checks, with sidelight options.
Sliding Door Calculator
Sliding patio door sizes, costs and U-value compliance.
First Fix: Electrics
Extension Electrical Calculator
Circuits, cable sizing, MCB selection and consumer unit spare ways.
Cable Sizing Calculator
Cable sizes for new circuits from the consumer unit.
Socket Spacing Calculator
Plan socket layouts for the new rooms.
Lighting Layout Calculator
Lighting positions and lux levels.
Maximum Demand Calculator
Check your supply can handle the additional load.
Part P Notification Calculator
Whether the electrical work is notifiable, and the certification routes.
First Fix: Plumbing and Heating
Extension Heating Calculator
Heat loss for the new rooms, radiator or UFH sizing, and boiler capacity check.
Radiator Sizing Calculator
BTU output needed for new rooms.
Boiler Size Calculator
Check your boiler capacity for the additional rooms.
Underfloor Heating Pipe Calculator
Pipe lengths if installing wet underfloor heating.
Pipe Sizing Calculator
Pipe sizes for hot and cold water supply extensions.
Insulation, Ventilation and Part L
U-Value Calculator
Check wall, floor and roof U-values meet Part L.
Insulation Thickness Calculator
Determine insulation depth for walls, floors and roof.
Cavity Wall Insulation Calculator
Cavity wall insulation quantities for the new walls.
Heat Loss Calculator
Heat loss for the new rooms to size the heating system.
Part L Compliance Calculator
Fabric U-values, air permeability and thermal bridging against Part L targets.
Extension Ventilation Calculator
Part F trickle vents, extract rates and purge ventilation for the new rooms.
Plastering and Finishing
Plasterboard Calculator
Plasterboard for walls and ceilings in the new rooms.
Plaster Coverage Calculator
Plaster bags needed for bonding and skimming.
Skim Coat Calculator
Multi-finish for skim coating all surfaces.
Jointing Compound Calculator
Tape and compound if joints are being filled rather than skimmed.
Paint Coverage Calculator
Paint quantities for decorating the new rooms.
Flooring
Frequently Asked Questions
All calculators are free to use with no signup required. Building an extension requires building regulations approval and may need planning permission. Always engage a qualified architect or architectural technician for design, and a structural engineer for beam and foundation calculations.