Plan your extension or conversion.
Planning an extension or conversion starts with understanding the space you want to create and the budget it may need. TNCO's Extension & Conversion Cost Calculator helps you explore house extensions, loft and garage conversions, garden rooms and conservatories.
Choose the project types you are considering, then adjust the size and available options to build an indicative starting budget. Select the UK or Ireland and review the priced scope shown for your location. You can explore several parts of a project together while keeping their individual scopes visible.
Review the calculation basis, specification and allowances alongside the headline range. The estimate helps you compare your ideas and decide which questions to bring to a design discussion. If you are also refreshing existing rooms, use our Renovation Cost Calculator for those works. TNCO can help develop your brief and turn the initial budget into a more defined project.
Calculator questions
Which projects can I estimate?
Explore house extensions, loft conversions, garden rooms, conservatories and garage conversions. Select the UK or Ireland, then a region, to see the available priced scope and budget currency. The calculation basis explains which work and allowances contribute to your selected estimate.
How should I measure an extension or conversion?
Use the outside width and depth for a house extension. The calculator works out its footprint and total new area for the selected number of storeys. Enter the loft floor area you plan to convert. Follow the size guidance shown for each project.
Can I plan an extension and conversion together?
You can select several project types and review their priced parts together. Works and design allowances retain their individual scopes, with one reserve applied to the combined works and extras. An extension and a conservatory are alternatives for the same rear space, so choose one of those.
What costs and allowances are included?
The result shows priced building work including modelled VAT, selected extras and the reserve you choose. Design and approval allowances appear separately when included. The reserve applies to building work and extras. Review each project's calculation basis for its fittings, service connections and any further scope to plan with TNCO.