Cost of a 5-metre dormer
Five metres is substantial. You utilize almost the entire width of the house and get a space that is almost equivalent to a full floor in terms of daylight and headroom. Structurally, however, a lot changes at this width: a steel profile is standard, a structural engineer is usually mandatory, and the crane is almost always larger. On this page, we explain where the money goes and which items you explicitly want to see in the quote.
We don't quote rates — we explain how the price is structured.
What belongs in a complete quote?
A good quote for 5 metres includes:
- Structure: trimmer construction with a steel profile as the main beam, calculated by a structural engineer.
- Exterior: roofing (EPDM, zinc, or class III bitumen), side walls, fascia board.
- Window frames: usually four window frames — combinations of fixed and tilt-and-turn.
- Insulation: roof Rc ≥ 4,7, side walls and possibly floor Bbl-compliant.
- Interior finishing: plasterboard, plastering, wide windowsill, painting.
- Installation: larger crane, demolition of existing roof section, debris removal.
- Structural engineer's calculation and possibly steel specification.
Request the steel specification and the calculation — a quote without substantiation is a red flag.
Why is the price-per-metre so favourable?
The fixed costs of a dormer — crane, setup, demolition, disposal, finishing — are spread over twice the width at 5 metres compared to 2,5 metres. Material and labour do scale up, but not proportionally. In practice, you see that the price-per-metre at 5 metres can be 25–35% lower than at 2 metres — while you get 2,5 times as much space.
This makes 5 metres attractive if you have the width and it fits structurally — but you pay for it with a heavier structure and a sturdier crane.
Steel profile and structural engineer — not optional
With a span of 5 metres, the entire weight of the walls, window frames, roof, and overlying roof parts rests on the horizontal main beam. Timber rarely suffices here — a steel HEB or HEA profile is standard.
A structural engineer makes the calculation and provides a steel specification with dimensions, quality, and fastening details. Count on a separate fee for the calculation and, if planning permission is required, for submission to the municipality. These items should be listed separately on the quote, not hidden under "miscellaneous".
Layout of four window frames
Multiple layouts are possible at 5 metres:
- 4× tilt-and-turn — maximum ventilation, highest costs.
- 2× tilt-and-turn + 2× fixed — popular and balanced.
- 1× tilt-and-turn + 3× fixed — minimal ventilation, greatest light output.
The more tilt-and-turn windows, the more expensive. With large glass areas, solar load plays a role: consider sun-control glazing or exterior sun shading, especially on the south and west.
Planning permission and aesthetic review at 5 metres
On the rear roof of a single-family home, 5 metres is often permit-free provided it complies with the Bbl: at least 0,5 m above the roof eaves, no higher than 1,75 m, and sufficient distance from the sides. However:
- In a protected cityscape, the permit-free status lapses.
- Some municipalities apply additional aesthetic review (welstand) requirements.
- On the front roof, planning permission is almost always required, along with an aesthetic review.
When planning permission is required, count on municipal fees, drafting work, and possibly an aesthetic review advice.
Comparison table
| Aspect | 3 metres | 5 metres |
|---|---|---|
| Window frames | Usually 2 | Usually 4 |
| Structure | Timber | Steel (HEB/HEA) |
| Structural engineer | Not always | Almost always |
| Crane | Standard | Almost always larger |
| Price-per-metre | Average | Significantly more favourable |
| Construction time | 5–7 working days | 7–10 working days |
Frequently asked questions
Short, honest answers to recurring questions.
- How much does a 5-metre dormer cost?
- We do not quote rates. The price depends on material, window frame type, glass, steel profile, structural engineer, accessibility, and planning permission.
- Does 5 metres always require planning permission?
- No. On the rear roof of a single-family home, it is usually permit-free, provided it complies with the Bbl and there is no protected cityscape.
- Is steel always necessary at 5 metres?
- Almost always. Timber rarely spans 5 metres without unacceptable deflection.
- Is a structural engineer included in the price?
- Sometimes yes, sometimes separately. Ask for it in writing. Count on a distinct fee item.
- Which roof is advisable at 5 metres?
- Class III EPDM or zinc — both are highly durable and suitable for low pitches.
- How many window frames fit?
- Usually four. Three wider ones are also possible, which provides larger glass areas.
- Do I need sun shading?
- Highly recommended for south and west orientations, otherwise the room will quickly become uncomfortable.
- How long does installation take?
- Prefab: 1 day of installation + 4–5 days of finishing. Traditional: 7–10 working days.
- What if the municipality imposes extra requirements?
- That happens in protected areas. Expect extra drafting work and possibly an aesthetic review advice.
- Is planning permission always expensive?
- Municipal fees vary per municipality; together with drafting work and a structural engineer, the process costs can become a serious item.
Summary
Five metres is large but cost-efficient per linear metre. The structure changes fundamentally: steel is standard, a structural engineer is usually mandatory, and the crane is larger. Therefore, demand a quote with the steel specification, structural engineer's calculation, and crane listed separately. If planning permission is required, count on municipal fees and drafting work. Compare at least three quotes based on identical specifications.