An extension is the most far-reaching way to gain space without moving house: a larger kitchen-diner, an extra room on the ground floor or a workspace at the side. In 2026 a finished extension costs on average €2,000 to €3,500 per m², including VAT and excluding kitchen or furnishings. For a typical extension of 3 by 6 metres (18 m²) that means €36,000 to €63,000.
The range is wide because an extension is really a small piece of new build: foundations, walls, roof, window frames, installations and finishing all come back, but on a small floor area. As a result, fixed costs (structural engineer, drawings, crane, scaffolding) weigh relatively heavily, and a 12 m² extension is more expensive per square metre than a 20 m² one. Below you can read what the different types of extension cost, where the money goes, what the permit rules are and which preparation and neighbour-related costs you should include in your budget.
Guide prices per type of extension
| Type | Guide price per m² (incl. VAT, finished) | Example |
|---|---|---|
| Simple rear extension (flat roof, standard window frames, plaster and floor) | €1,800 – €2,500 | 12 m²: €22,000 – €30,000 |
| Standard rear extension (large glazing, underfloor heating, roof light) | €2,500 – €3,500 | 18 m²: €45,000 – €63,000 |
| Luxury extension (full-width sliding doors, bespoke work, high-end finish) | €3,200 – €4,500 | 20 m²: €65,000 – €90,000 |
| Side extension (detached or semi-detached house) | €2,200 – €3,800 | 15 m²: €33,000 – €57,000 |
| Conservatory or garden room (lots of glass, light foundations) | €1,500 – €3,000 | 12 m²: €18,000 – €36,000 |
| Upward extension (extra storey on an existing extension or garage) | €2,500 – €4,500 | 20 m²: €50,000 – €90,000 |
A rear extension is the most common and the cheapest per m², because the rear wall often already has a large opening and the site is easy to reach via the garden. A side extension more often needs a new load-bearing structure and changes to the existing wall. A conservatory looks cheap, but an unheated or poorly insulated conservatory is too hot in summer and too cold in winter; a well-insulated glass garden room quickly costs as much as a brick extension. An upward extension is the most expensive per m²: the existing structure often has to be reinforced, and an upward extension is never permit-free.
Where the money goes
For a standard extension of 18 m², the breakdown looks roughly like this:
| Component | Share | What it includes |
|---|---|---|
| Preparation | 5 – 10% | Drawings, structural engineer, soil survey, municipal fees, possibly a kwaliteitsborger (independent quality assurer) |
| Foundations and groundwork | 10 – 15% | Excavation, ground beams or strip foundation, possibly driven piles or screw piles |
| Shell | 20 – 25% | Concrete floor, cavity walls or timber frame, insulation, opening up the rear wall with a steel portal frame |
| Roof | 10 – 15% | Roof structure, insulation, EPDM or bitumen, roof light, roof edge, rainwater drainage |
| Window frames and glazing | 10 – 20% | Sliding doors or French doors, HR++ or triple glazing, sun shading |
| Installations | 10 – 15% | Electrics, heating or underfloor heating, ventilation, extending the central heating or connecting a heat pump |
| Finishing | 15 – 20% | Plastering, flooring, skirting boards, painting, exterior finish and restoring paving |
Two items often come as a surprise. The foundations: on clay or peat soil (large parts of the west and north of the Netherlands) piles are needed, which costs €4,000 to €10,000 more than a shallow foundation. And opening up the rear wall: a steel beam with columns to carry an opening of 5 to 6 metres costs €3,000 to €7,000 including installation and making good. Both items can only be budgeted properly after a cone penetration test and a structural calculation; a quote that is not based on these is an estimate. What is involved in removing a load-bearing wall is explained in Removing a load-bearing wall: rules and step-by-step plan.
Permit-free or not?
Under the Omgevingswet (Environment and Planning Act), a rear extension is often permit-free if it stays within the achtererfgebied (the rear garden zone) and meets the size limits:
- no more than 4 metres deep, measured from the original rear wall;
- no more than 5 metres high for a single-storey extension with a flat roof, and no higher than 30 centimetres above the first-floor level;
- the total of outbuildings and extensions stays below the permitted building coverage (for a plot up to 100 m²: at most 50 per cent);
- the home is not a listed building and is not in a protected townscape or village conservation area.
Permit-free does not mean rule-free: a permit-free extension must also comply with the Besluit bouwwerken leefomgeving (the Dutch building regulations: insulation, ventilation, structural safety) and with neighbour law. An extension at the front or side bordering public space, or an upward extension, requires an omgevingsvergunning (environmental permit). Deeper than 4 metres usually requires a permit, unless the extension is no higher than 3 metres and stays within the bebouwingsgebied (the zone around the house where permit-free building is allowed). Municipalities can deviate from this in their omgevingsplan (local zoning plan), so always do the permit check on the Omgevingsloket (the government permit portal) and have the contractor or draughtsperson check the bebouwingsgebied calculation; the definition is precise. Read more in When do you need a permit for a renovation.
Drawings, structural engineer and municipal fees
Even without a permit you need drawings: the contractor does not build from a sketch, and without working drawings you cannot compare quotes. Allow for:
- Architectural draughtsperson or architect: €1,500 – €4,000 for design and working drawings; an architect who guides you through the whole process charges 8 to 12 per cent of the building cost.
- Structural engineer: €600 – €1,500 for the calculation of foundations, portal frame and roof.
- Soil survey (cone penetration test): €500 – €1,500, needed to determine the foundations.
- Leges (municipal fees) for a permit application: municipalities usually charge 1 to 3 per cent of the building cost, with a minimum. For a €50,000 extension that quickly comes to €750 to €1,500.
You incur most of these preparation costs before you have chosen a contractor. See them as an investment in a reliable quote: a contractor who can price from complete drawings and a structural calculation needs to build in less for contingencies.
Wkb: quality assurance for an extension that needs a permit
The Wet kwaliteitsborging voor het bouwen (Wkb, the Dutch Quality Assurance in Construction Act) has applied since 1 January 2024 to new build in consequence class 1. For renovations, including an extension, the introduction of the system has been postponed and there is currently no final start date. Once the system also applies to renovations, you will need to hire an independent kwaliteitsborger (quality assurer) for an extension in consequence class 1 that requires a permit. They check during construction whether the work meets the building regulations and, on completion, deliver a declaration and file to the municipality. Because the system does not yet apply to renovations, there is no market price yet; allow for a few thousand euros. A permit-free extension does not need a kwaliteitsborger. Check the current status when you apply, because the timetable has shifted several times in the past.
What already applies: the contractor's liability has been tightened since 2024 for contracts concluded from that date. They remain liable for defects that were not discovered at handover, unless they can show these are not attributable to them. More on this in The Quality Assurance Act (Wkb): what it means for your renovation.
Neighbours: inform, agree, record
An extension almost always affects the neighbours: less light, building up to the boundary, a crane over their garden, or a shared wall in a terraced house. Legally you may build up to the boundary, but windows that look out onto the neighbours' plot must stay at least 2 metres from the boundary, unless the neighbours give written consent. If you want to build against a shared (mandelige, jointly owned) wall, you need permission. Inform the neighbours before the quote stage, show them the drawing and record agreements about working hours, boundary fencing and any damage in writing. A pre-construction building survey of the adjoining home (€300 – €600) prevents arguments about cracks afterwards: without a baseline record, every crack the neighbours find later will be attributed to your renovation. See also Renovating and the neighbours: rights, obligations and good agreements.
How long does it take?
- Design and preparation: 6 to 12 weeks (drawings, structure, quotes).
- Permit (if needed): 8 weeks for the regular procedure, plus a 6-week objection period.
- Construction: 6 to 10 weeks for a standard extension; timber frame or prefab elements are faster than brickwork, partly because of shorter drying times.
- Contractor waiting time: often 2 to 6 months in 2026.
From first idea to use, six months to a year is realistic. Start preparing in the autumn if you want to build in the spring, and only ask the contractor for a start date once the drawings and the structural calculation are final. A bricklayer or carpenter acting as main contractor can be a good alternative to a building firm for a simple extension; in that case, check who coordinates the installations and the finishing.
What does an extension deliver?
Extra living space increases the value of the home, but rarely one-for-one with the investment: the added value is usually clearly lower than the building costs, higher in areas with housing shortages and with a well-integrated kitchen-diner. The biggest gain is often indirect: you avoid a move with transfer tax, estate agent fees and a more expensive mortgage. Include insulation, underfloor heating and preparation for a heat pump straight away during construction; that is cheap during the build and expensive afterwards. How to finance the build, with a bouwdepot (construction deposit account), a mortgage increase or savings, is explained in Financing a renovation: construction deposit, renovation mortgage or savings.