How wide may a dormer be?
For a permit-free dormer on the rear roof plane, the Dutch Building Decree (Bbl) does not prescribe a maximum width, but it does require: at least 0.5 m clearance to the sides, ridge and gutter. In practice your roof plane determines the maximum width. The front roof plane always requires a planning permit and is subject to local welstand rules.
The legal minimum: 0.5 metres clearance on all sides
For permit-free dormers on the rear roof plane the Bbl requires a minimum 0.5 m clearance to the sides of the roof plane, to the roof ridge (top) and to the gutter (bottom). The dormer height may be a maximum of 1.75 m, measured from the foot.
How to determine the maximum width of your roof
Measure the width of your rear roof plane (from one bend/side ridge to the other) and subtract 1 metre (2 × 0.5 m). That is the maximum dormer width on the outside. Note that you still need 0.5 m to the ridge and gutter — these do not affect width but do affect placement.
Front roof plane: always permit, local rules
On the front roof plane local welstand rules apply and an omgevingsvergunning is always required. Many municipalities apply rules of thumb such as 'no more than 50% of the roof plane width' or 'no more than 70%'. Consult your municipality's welstand policy.