External and internal dimensions answer different questions
External dimensions help position the extension on the plot. Inside, the space between finished walls matters. Our simple model subtracts two side walls and one garden wall, leaving the house connection open. Actual wall build-up, columns and junctions may differ.
Three examples with 30 cm walls
| External dimensions | Approximate internal dimensions | Approximate internal area |
|---|---|---|
| 5 × 3 m | 4.40 × 2.70 m | 11.88 m² |
| 6 × 3 m | 5.40 × 2.70 m | 14.58 m² |
| 6 × 4 m | 5.40 × 3.70 m | 19.98 m² |
Furniture needs more than its footprint
A two-metre table also needs room for chairs to pull out. Kitchen cupboards and a dishwasher must open while someone passes. Draw open components on the plan. A strip of floor beside the table is useful only if everyday movement fits.
Draw what stays in the original room
The extension is part of your house. Remaining wall sections, radiators or doors may determine the usable layout. Review the whole ground floor, not just the new rectangle. Moving the connection can sometimes be more useful than adding depth towards the garden.
When is a formal measurement needed?
Our calculation supports early planning and furniture layouts. For sale or valuation, use a measurement of the finished home under the applicable guidance. NVM explains what counts towards usable floor area. Keep the measurement with the final drawings.