How much living space does your extension add?

Start with external dimensions, design around the interior

The configurator uses external dimensions to define the extension’s footprint. This helps with initial planning, but your sofa, table or kitchen must fit inside the walls. A 600 cm width with two 30 cm side walls leaves 540 cm internally in a simple example. The actual construction may be thinner or thicker.

For a rectangular extension opening into the house, our calculator deducts one front wall from the depth. A 300 cm external depth becomes 270 cm internally. This gives 5.40 × 2.70 = 14.58 m². Recesses, offsets, columns, the existing rear facade and connection details are not resolved in this simple estimate.

Have the living area measured

Consistent measurement matters for sale and valuation. The Dutch industry measurement instruction sets out which parts belong in each area category. The internal dimensions from our calculator are not a survey report. Have the classification checked when uncertain, especially for conservatories, storage rooms and detached outbuildings.

Make the extra area work in everyday life

Draw furniture to scale and allow room to pull chairs out, open doors and pass one another. Include radiators, curtains, cupboards and the clear door opening. A long narrow room can have enough square metres yet be difficult to furnish. Compare different proportions, such as a wider and shallower plan.

Check how much patio and garden remain. Temporarily mark out the footprint with string before finalising the design, so you can experience the new facade position. This practical check does not replace a survey, but helps connect dimensions on paper with your own home.

Which dimensions should the quote show?

• External width and depth, and the position relative to the existing facade.
• Wall thicknesses, clear internal dimensions and finished clear height.
• Dimensions of the wall opening and the actual clear door passage.
• Floor level, thresholds and any steps between the existing and new space.

Always compare the same type of floor area

Label each area as external, internal or measured living area. The simplified three-wall model helps with furniture planning but is not an official floor-area measurement. For sale or valuation, have the finished space measured under the applicable guidance.