Bbl requirements for your dormer (formerly Building Code)
The Bbl (since 2024, successor to the Bouwbesluit) sets minimum requirements for new build and renovation, including dormers. Permit-free doesn't mean code-free; you remain responsible. This page lists the numbers your contractor must meet.
- Roof R-value (renovation)
- Min 4.7 m²K/W
- Roof R-value (new build)
- Min 6.3 m²K/W
- Glass U-value
- Max 1.2 W/m²K (HR++)
- Daylight area
- Min 0.5 m² per habitable room
- Fire resistance to neighbours
- 30 min WBDBO
Insulation and energy
Renovation requirement for roof R-value: at least 4.7 m²K/W when extending an existing house. New-build requirement: 6.3 m²K/W. Many installers default to R 4.5 (the old value); ask explicitly for R 4.7 minimum and have it in the quote.
Glass: at least HR++ (U-value 1.2 W/m²K or better). Premium: triple glazing (U 0.6-0.8). With triple glass check frame capacity — triple weighs 50% more than HR++.
Daylight and ventilation
Equivalent daylight area: at least 0.5 m² per habitable room. A 3-metre dormer with two tilting windows typically delivers 1.5 m² net window area — comfortably above.
Ventilation: at least 0.9 l/s per m² of habitable floor via frame grilles or mechanical vent. For an 8 m² attic room that's 25-30 m³/hour; standard self-regulating grilles usually suffice.
Structure and fire safety
Load: 1.5 kN/m² for habitable space (150 kg/m²). The existing roof must demonstrably support extra weight (200-400 kg/m for prefab); from 4-metre dormers a structural calc is mandatory at permit application.
Fire resistance to neighbours: 30 minutes WBDBO. On terraces this requires a gypsum-board layer inside (minimum 12.5 mm Type F fire-rated) or a rockwool fill with double-sided board finish.
Sound and moisture
Sound insulation outside-inside: minimum 20 dB(A) characteristic index. Along a rail line or busy road, often 25-33 dB; check the municipal noise map.
A vapour barrier is mandatory inside the insulation (film or OSB with taped seams). Without it moisture condenses inside the insulation and you get mould and timber rot within 2-5 years.
FAQ
- Who checks Bbl compliance?
- For permit-required dormers the municipality checks upfront and spot-checks during build. For permit-free, no one; you bear responsibility. Ask your contractor for a written declaration.
- Can I undercut Bbl on a permit-free dormer?
- No, the Bbl always applies. At later complaints or sale, the owner pays for bringing-to-spec.
- What R-value is sensible above minimum?
- R 6.0-7.0 is a good investment: €15-€25/m² extra insulation cost, repaid in 5-8 years via lower heating bills.
- Is a ventilation grille enough?
- For a bedroom yes, if 25-30 m³/hour capacity. For a bathroom or kitchen in the dormer, mechanical extraction is required.
- Should I redo the energy label?
- Not required, but smart at sale time. A dormer with R 6+ can shift the label one step (B to A), worth 3-5% in property value.