1. Choose one comparable size first
Record width and depth as external dimensions. A 6 by 3 metre extension covers 18 m² externally but offers less usable space because of its walls. Keep doors, roof and finishes equal when comparing quotations, or one figure may describe a simple shell and another a larger package.
2. Read the size price and allowance separately
These amounts use the calculation model’s recorded price references, updated on 13 September 2026. No extra options are selected. Our standard €1,500 transport allowance is added. The subtotal is a worked example, not a quotation or confirmation that every task at your property is included.
| External size | Base reference | With transport allowance |
|---|---|---|
| 5 × 3 m | €47,316 | €48,816 |
| 6 × 3 m | €51,991 | €53,491 |
| 6 × 4 m | €61,141 | €62,641 |
3. Work from essential to optional
Add options required by your intended use first. A bedroom has different layout and privacy needs from a dining room. Then compare facades and comfort upgrades. Save each previous design so you can see the difference and reconsider without starting again.
4. Check work beyond the extension itself
Discuss ground investigation, existing services, the structural opening and repairs inside. Ask who lifts and reinstates paving. These define the complete job and should be resolved before ordering. Identify fixed prices, allowances and exclusions, and do not add work twice when it is already demonstrably included.
Your next step
Create a base design and a preferred design. Bring both cost breakdowns and a list of work to the existing house to the discussion. This gives your quotation a consistent starting point.