Plan an extension
Measure your space, choose your build details, then work out your materials.
Start with your measurements
Use a tape measure or your drawings. Already know the area? Use the quick-entry option.
Room or surface
Surface name or level optional
Leave out a measured opening optional
Add it to this surface only. A wall window never reduces your floor.
Use the actual surface measurements, not a guessed size.
This matters when adding several sections. Footprint and floor area are separate; upstairs floors do not add ground footprint.
Roof, floor & heating
Choose what you know. Leave the rest undecided.
More about your extension optional
Work out your materials
Choose a material. Use your measurements, then enter the details of the product you have chosen.
Choose a material to start. Turn on JavaScript to use the calculator.
What this can and cannot work out
Quantities use the measurements and product facts you supply. Your project team must specify foundations, structural floors, roof construction and heating. This is not a construction design or suitability check. Keep different products and layers in separate estimates.
Your budget optional
Add quotes or allowances you already have. You can measure first if you do not have prices yet.
Quote or allowance
Your own quote or allowance totals appear here. No market prices are assumed.
Keep the professional decisions visible
Read the extension permissions guide →
Planning, building requirements and private rights are different checks. This is not an approval checker.
Your tasks and dates
These are your notes, not a guaranteed programme. Record specifications, planned dates and anything not applicable.
Agree your scope and budget
Record what the builder’s quote includes and excludes.
Survey and professional design
Ground, existing building, structure, lintels and opening supports need project-specific design and measurements.
Usually follows: Agree your scope and budget. Adjust your own dates with your project team.
Permissions and private rights
Confirm the requirements with your local authority and relevant professional.
Usually follows: Survey and professional design. Adjust your own dates with your project team.
Foundations and structural work
Use the designer’s specification. This planner does not choose depths, reinforcement or steel.
Usually follows: Permissions and private rights. Adjust your own dates with your project team.
Walls, roof and weatherproofing
Confirm the specified wall leaves, roof structure, glazing and weatherproofing.
Usually follows: Foundations and structural work. Adjust your own dates with your project team.
Drainage and damp protection
Record the professional drainage and floor/wall build-up decisions.
Usually follows: Survey and professional design. Adjust your own dates with your project team.
Electrical, plumbing, heating and ventilation
Use competent project-specific design and installation; quantities do not size these systems.
Usually follows: Walls, roof and weatherproofing. Adjust your own dates with your project team.
Floors, lining and insulation
Record the specified floor layers, insulation and wall/ceiling lining. If you plan underfloor heating, keep the confirmed heating system, floor build-up and chosen finish together with your installer.
Usually follows: Electrical, plumbing, heating and ventilation. Adjust your own dates with your project team.
Kitchen, bathroom and decoration
Mark work not applicable if it is outside your scope.
Usually follows: Floors, lining and insulation. Adjust your own dates with your project team.
Inspections and completion documents
Confirm required inspections, commissioning and applicable certificates.
Usually follows: Kitchen, bathroom and decoration. Adjust your own dates with your project team.
Your extension plan
Calculate your current areas and budget to review them. Unpriced work and professional decisions are not treated as zero.
One extension plan is saved privately in this browser only. This does not add anything to My Materials List or send measurements to a supplier.
Where the numbers come from
From extension budget to material quantities
Your budget uses quotes, allowances and VAT facts you enter. It is not a whole-build price calculated from material packs. Measure ground footprint, individual floors, walls and roof faces separately; windows belong only to their owning wall or roof face.
Can I start before choosing materials?
Yes. Areas and a record of outstanding decisions are useful on their own. Product quantities need compatible coverage, yield or packing details; missing facts are shown, not guessed.
Does this tell me what is allowed or safe to build?
No. Use your local authority and competent professionals for permissions, structure, foundations, roof, drainage and services. Record that information alongside your planning estimates.
Your materials list stays separate
This Project keeps separate estimates in your private extension plan. It does not create an aggregate My Materials List recommendation or add items automatically.
Can I keep the plan?
Choose Save on this device, copy a summary or print. No account, cloud calculation or synchronisation is involved. Browser site-data clearing removes the saved plan.
Browse individual calculators · Extension permission guidance