Calculator
What could extra m² add in value?
Enter your municipality, type of home and how many m² you would like to add. You immediately see an indication of what an extension or roof extension could add in value, based on real 2026 sale prices per m². A direction, not a promise.
Possible added value
+ €70,000 – €99,000
What the extra m² could add in value
This is an indication of what is possible, based on average sale prices per m². It is not a valuation and not a promise: we do not guarantee your home will fetch this amount. The real value depends on your street, plot, finish and the market when you sell.
Discuss your extension or roof extensionHow we calculate
- 1
The basis: sale price per m²
We use the median transaction price per m² of living area that the NVM publishes each quarter, per type of home. An apartment in Amsterdam fetches far more per m² than a terraced house in Purmerend, which is why you choose both.
- 2
Adjustments: size, condition and label
A large house fetches slightly less per m² than a small one of the same type, because land and location are paid for once. We then adjust for condition (−10% to +10%) and energy label (−5% to +4% relative to label C).
- 3
Extra m²: 85% of the local price per m²
An extra square metre is worth less than an average one, because you extend the house, not the plot or the location. We use 85% of the adjusted price per m², with a range of 70% to 100%. A well-designed, light-filled extension tends towards the top; a dark or awkward one towards the bottom.
Sources
- NVM housing market quarterly figures, Q1 and Q2 2026: municipality of Amsterdam, Amsterdam-Weesp, municipality of Purmerend, Zaanstreek region and the Netherlands. We use the average of both quarters, because one quarter in a small municipality is only a few dozen sales.
- For municipalities the NVM does not report separately, we scale the Purmerend profile to that municipality’s average price level (Huiswaardemeter, October 2026). The result says so when that applies.
- We update the figures when new quarterly figures are published.
What does an extension cost? · What does a roof extension cost?
Questions about the calculator
Is this the same as a valuation?
No. A valuer looks at your house, the plot and recent sales in your street. This calculator works with averages per municipality and type of home. For a mortgage or a sale you always need a validated valuation report.
Is this amount a promise?
No. It is an indication of what is possible, based on average sale prices in your municipality. We do not promise that your home will fetch it, and the result is not a valuation. What a buyer eventually pays depends on your home, the street and the market at that moment.
Which floor area should I enter?
The usable living area: the interior of the home, excluding a shed, garage or attic that does not count as living space. You will find it in the sales brochure or via the WOZ-waardeloket.
Will I earn an extension back when I sell?
That depends on the building costs in your situation and how long you stay. The calculator shows the estimated added value, not a building price. The pages on the cost of an extension and a roof extension explain what that price depends on.
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