Why one square-metre price is not enough
Larger extensions use more material, but many preparations are similar. A compact project may therefore cost more per square metre. Groundworks, lifting and wall openings do not always scale with the floor area.
Compare quotations for the same external dimensions, quality and finishes. A low total means little if foundations, electrics or plastering are missing.
How your estimate is calculated
The extension configurator uses 78 recorded size prices from De Prefabriek’s public configurator. Prices between those observations are calculated. The base, selected options and transport allowance appear separately. Transport starts at an adjustable €1,500 allowance: our own budgeting assumption, not a confirmed delivery charge.
All displayed amounts include VAT. Each change recalculates the total, including facade choices, underfloor heating, plastering or an extra crane. The separate budget mode uses your chosen amount per m². It is for your own scenario and does not constitute a supplier quotation.
Items to check separately
- Foundation type, ground investigation and any piles.
- Structural calculations, drawings, permits and municipal fees.
- Crane, transport, traffic management and temporary facilities.
- Changes to existing pipes, electrical board, kitchen and heating.
- Floor finishes, garden reinstatement and work on the existing house.
From estimate to a firm price
After the survey, assumptions become a project specification. Request an overview of included work, exclusions and provisional allowances. Also agree how changes are priced and when payments are due.
Keep room in your budget for decisions still to be made. Discuss financing with your lender or an independent adviser.
Create one budget for the whole project
Put every cost item in one overview, even when different businesses do the work. Separate preparation, shell construction, site work, interior finishing and external work. Mark each amount as fixed, provisional or missing. This avoids counting items twice or assuming a connection is included elsewhere. Confirm whether prices include VAT and compare figures on the same basis.
Save without leaving out essential work
First compare area, facade finishes and extra glazing. A simpler design may need fewer complex junctions. Then identify finishing work you might do yourself and include its time, materials and equipment. Do not remove structural safety, waterproofing or essential connections just to display a lower total. A saving is meaningful only when the required function and quality are preserved.
Building costs are not the same as added value
A building budget shows the project cost; a valuation considers what buyers may pay for the home. These figures can differ. Use a complete budget alongside the value calculator. A positive difference is not a profit guarantee, and a negative difference does not mean the extension fails to improve daily life. Separate financial considerations from personal choices.
Compare a base design with your preferred design
Save a design without extra options first. Add the features you will use daily and save a second version. The difference shows the cost of those choices. Then check the same site works for both: a simpler facade does not automatically make foundations or structural openings cheaper. Discuss any omitted work explicitly, even when the configurator displays a complete total.