Before planning an extension, check the rules for your UK nation and your property. Calculator.Shop estimates material quantities; it does not decide whether work is permitted, compliant or exempt.
Separate checks, not one approval
Planning permission concerns what you build and where. Permitted development is a planning route with conditions, not a blanket go-ahead. Building regulations cover technical matters such as structure, fire safety and energy performance; Scotland uses building standards and building warrants.
Party walls and boundaries are separate again. The Party Wall etc. Act 1996 applies in England and Wales, not Scotland or Northern Ireland. Shared walls, boundary work and nearby excavation need a review of ownership and neighbour rights. Read the official Party Wall Act scope guidance.
Site restrictions can change the planning route. Check listed status, conservation or other protected areas, previous extensions and any local conditions or withdrawn rights. Planning permission does not settle private ownership or access rights.
England
Use England’s householder rules. Whether an extension can use permitted development depends on the property, its history and the whole proposal; measurements alone do not decide it.
- Planning: check property type, original-house history, extension position and any prior-approval route.
- Site restrictions: check listing, protected areas, Article 4 directions and planning conditions.
- Building regulations: confirm the approval route separately from planning.
- Party wall: review shared walls, boundary work and excavation near neighbours.
Official guidance (opens in new tabs):
- Householder permitted development — GOV.UK
- When planning permission is required — GOV.UK
- Building regulations approval — GOV.UK
- Party walls and building work — GOV.UK
Sources checked: .
Wales
Use Welsh extension rules, not England’s allowances. Permitted development depends on the extension type, original house and site restrictions; building regulations remain a separate check.
- Planning: check Welsh rules for the proposed storeys and position, plus previous extensions.
- Site restrictions: check property type, listing, protected areas and withdrawn development rights.
- Building regulations: confirm Welsh requirements independently of planning.
- Party wall: review shared walls, boundary work and excavation near neighbours.
Official guidance (opens in new tabs):
- Planning permission: extensions — Welsh Government
- Building regulations: extensions — Welsh Government
- Party walls and building work — GOV.UK
Sources checked: .
Scotland
Use Scotland’s dwellinghouse extension guidance. Permitted development and planning permission are separate from the Scottish building standards and building-warrant process.
- Planning: check dwellinghouse status, extension storeys, siting and previous development.
- Site restrictions: check conservation areas, listing, local restrictions and landowner permission.
- Building standards: confirm the building-warrant requirements before starting work.
- Shared walls and boundaries: check Scottish ownership and neighbour rights, not the England/Wales Party Wall Act.
Official guidance (opens in new tabs):
Sources checked: .
Northern Ireland
Use Northern Ireland’s planning rules. Extension permitted development has conditions; council building control and private property rights need their own checks.
- Planning: check property type, extension conditions and the original planning permission.
- Site restrictions: check listing, conservation areas and Areas of Outstanding Natural Beauty.
- Building regulations: confirm the council building-control requirements separately from planning.
- Boundaries: check covenants, leases and neighbour rights; the England/Wales Party Wall Act does not apply.
Official guidance (opens in new tabs):
- Planning permission: when to apply — nidirect
- Building regulations approval — nidirect
- Before you start work: private rights — nidirect
Sources checked: .
Get the design before the quantities
Use a qualified designer or structural engineer for foundations and steel. Ground conditions, trees, nearby buildings and loads affect the design. A material estimate cannot choose foundation depth, beam sizes or temporary supports.
Roof and drainage details also need a professional design: supports, weatherproofing, insulation and rainwater routes must work together. Establish existing drain and sewer locations and check any sewer-owner requirements before fixing the layout.
Keep your material plan separate
Once you have professional dimensions and specifications, use Plan an Extension to organise quantity estimates. A saved estimate is not a design, planning decision or building-control approval.
Guidance checked on . We have not checked your address, property history, proposed drawings, title, local restrictions, ground conditions or services. Follow the official links and confirm the requirements for your own site before work; rules may change.