A dormer on a flat roof: what it really is

A dormer is by definition a protrusion from a pitched roof. On a flat roof there's no pitch, so the structure is by definition an addition. Different rules (zoning, height, fire), different permit, different contractors.

Usually tendered via a general builder with addition experience, not a dormer specialist. When in doubt: ask the municipality whether a specific plan falls under 'addition' or 'dormer' — this determines the application route.

Full new floor (if not present), timber-frame or steel-frame walls, roof slab with PVC or EPDM covering. Insulation per current regulation (BENG 2024: R ≥ 6.3 roof, ≥ 4.7 walls, ≥ 3.7 floor).

Heating via existing system or heat pump. Large additions often need a new central-heating circuit; plan €2,500-€5,000 in installation costs on top of construction.

Always permit-required, often zoning check too (can your house be taller?). Amsterdam, Rotterdam and other large cities have specific rules for additions above existing housing (max height, neighbour shading, fire compartmentation).

Expect 6-9 months permit process including structural calculations, energy-label update and possibly neighbour consultation. Architect mandatory above 1,000 m³ of addition.

Worthwhile: 1960s/70s houses with a flat roof and no attic, where you want structural expansion without going underground. Often delivers €30,000-€60,000 value uplift on €25,000-€45,000 investment.

Less worthwhile: small additions <8 m² (poor cost-benefit), or houses with limited foundation (structural reinforcement can double the project cost).