The trapezium dormer: subtler in the streetscape
A trapezium dormer has sloped cheeks following the roof pitch instead of vertical sides. Result: less bulky look, often approved in aesthetics-sensitive areas where a regular dormer would be rejected. Price €7,000-€14,500, €700-€1,500 above rectangular.
- Price 2026
- €7,000-€14,500 (2-5 m)
- Premium over flat
- €700-€1,500
- Cheek angle
- Typically 30-50° (matching roof)
- Interior loss
- 20-40 cm at sides
- Construction
- Mostly traditional custom
When trapezium is the only option
Some municipal aesthetics policies forbid 'box shapes' in protected village views. The trapezium avoids that: sloped cheeks reduce visual volume and match roof geometry.
Examples: Marken, Volendam, Giethoorn, parts of the Zaan area and the Veluwe villages. In these areas rectangular execution nearly always gets a negative aesthetics advice.
Interior space: similar or slightly less
The sloped cheeks cut 20-40 cm from the corners. A 4-metre trapezium dormer is 4 m wide at the widest and 3.2-3.6 m at wall level.
Fine for a study or play corner; less ideal for maximum bed area. A 1.80 m double bed fits sideways with room for nightstands; a sideways double bed with built-in wardrobes alongside becomes tight.
Production: usually traditional custom
Prefab makers don't carry trapezium shapes by default; almost always a traditional custom build. Build time 4-6 days, price €1,500-€2,500 above standard prefab.
Request quotes explicitly for 'trapezium' execution and lock the cheek angle in drawings up front. Some installers default to 35°; for a natural join with a 45° main roof, 45° cheeks work better.
Aesthetics process in practice
In protected village views, submitting a sketch plan beforehand pays off (€150-€300 at the municipality). The aesthetics commission gives a preliminary opinion — if positive, you can submit the full permit with confidence.
A direct permit application without pre-consultation has a 35-45% rejection rate in protected zones. With pre-consultation that drops to 10-15%.
FAQ
- Is a trapezium dormer structurally heavier?
- Not materially. Sloped cheeks are often lighter as they have less area than straight ones. Structural difference <5%.
- Can I DIY a trapezium dormer?
- Technically yes with skill, but corner joints and water details need roofer's craftsmanship. Not recommended for DIY builders.
- What angle do the cheeks usually have?
- 30°-50°, often equal to the main roof pitch for a natural join. 45° is a common default.
- Does trapezium suit a flat urban townhouse?
- Usually not. The shape belongs to steep tile roofs; on a 20° roof it doesn't read logically.
- How much more does trapezium cost than flat?
- €700-€1,500 for a prefab variant if available; €1,500-€2,500 for traditional custom.