Dormer or roof extension: which suits your house?

Dormer or roof extension: what is the difference?

A dormer is a projection built out from the sloping roof. The roof stays in place, but in one spot you get a vertical wall with a window and extra headroom. A roof extension replaces all or part of the roof with a new storey with straight walls. You then gain usable space across the full width and depth.

• Dormer: extra headroom in part of the attic
• Dormer: often installed in one to a few days
• Roof extension: a full extra storey
• Roof extension: more preparation, an engineer and a permit

What does a dormer cost and what does a roof extension cost?

A prefab dormer about 3 metres wide costs roughly €7,000 to €12,000, a wider or traditionally built dormer €12,000 to €18,000 or more. A roof extension on a terraced house costs roughly €50,000 to €90,000, depending on size and finish. These are indications including VAT and installation.

Per square metre of usable space the difference is smaller than those figures suggest, because a roof extension gives you much more floor with full headroom. How the costs of a roof extension are made up, you can read under what a roof extension costs.

Which permit do you need for a dormer or roof extension?

A dormer on the rear roof slope is generally permit-free if it meets the national conditions. Among other things: no higher than 1.75 metres, the bottom between 0.5 and 1 metre above the eaves, the top more than 0.5 metres below the ridge and the sides more than 0.5 metres from the sides of the roof slope.

A dormer at the front and a roof extension almost always need a permit. Since the Environment and Planning Act came into force on 1 January 2024, you do the check and the application via the Omgevingsloket. Your municipality's environment plan may differ, so always have it checked. The procedure is explained under roof extension permit.

When do you choose a roof extension instead of a dormer?

Choose a roof extension if you want to turn the attic into a full storey with several rooms, or if the roof is so low that little headroom remains even with a dormer. Choose a dormer if the attic is already fairly usable and you are mainly looking for light and some extra height.

Sometimes there is a third way: a clever layout of the existing attic with a storage loft above it. In Heerhugowaard we created a storage loft and two attic rooms for a family. Also read creating an attic room and extension or roof extension. In a no-obligation conversation we are happy to look at what suits your house.

Frequently asked questions