What does a 4-metre dormer cost?
A 4-metre dormer costs as an indicative range €7,000 to €10,000 including installation and VAT. Two or three frames, more living space than a 3-metre — but the price does not scale linearly: extra glass, wider structure and heavier trimmer make it relatively more expensive than 'one extra metre'. Roof pitch and height affect accessibility and thereby the install price.
Why 4 metres is relatively more expensive per metre
Fixed costs (transport, crane, scaffolding, prep, cleanup) stay the same, but materials scale up. Extra frame and glass, longer trimmers that must be heavier due to roof loads, longer gutter and roof trim, more EPDM/zinc on the roof, and more insulation. Expect 25-40% more than 3 metres — not 33% (as pure arithmetic would suggest).
What changes structurally vs. 3 metres
From 4 metres the trimmer work is often heavier or additional couplings between frames are added. Cheeks may be wider to keep the dormer visually proportional. Sometimes a side roof window is required for daylight/ventilation under the Building Decree. Ask the builder to explain this on the drawing.
Planning permission at 4 metres
The rear roof plane is usually permit-free provided the dormer stays at least 0.5 m from the sides and roof ridge and is no taller than 1.75 m. The front roof plane always requires an omgevingsvergunning. At 4 metres you usually still fit well within the rules on the rear roof, provided the roof plane is wide enough.
Items often forgotten
At wider sizes, the following are often forgotten: extra ventilation grids (to meet the Building Decree), electrical for two rather than one light point, and colour matching of the cladding with the existing facade. Include these explicitly in your quote request.