Installing a dormer yourself costs €5,000 to €12,000 in materials (including a prefab unit), depending on type, size and finish. Having a dormer installed by a specialist costs €10,000 to €25,000.
What Does a Dormer Cost? DIY vs. Contractor
Cost Comparison
| Dormer Type | Width | DIY (prefab + materials) | Contractor (all-in) | Saving |
|---|---|---|---|---|
| Small flat-roof dormer (prefab) | 150 cm | €5,000 – €8,000 | €10,000 – €16,000 | €5,000 – €8,000 |
| Standard flat-roof dormer (prefab) | 200–250 cm | €7,000 – €11,000 | €14,000 – €20,000 | €7,000 – €9,000 |
| Large flat-roof dormer (prefab) | 300+ cm | €9,000 – €14,000 | €18,000 – €28,000 | €9,000 – €14,000 |
| Lean-to dormer (bespoke) | 200–300 cm | €8,000 – €15,000 | €20,000 – €35,000 | €12,000 – €20,000 |
Types of Dormers: Which Suits Your Home?
Flat-Roof Dormer
The most common type. Rectangular box with a flat roof (minimum fall). Suits most properties.
Prefab cost (200 cm wide): €4,000 – €8,000
Shed Dormer (Lean-to)
Dormer with a sloping roof continuing in one plane from the existing roofline. Architecturally attractive.
Prefab cost (200 cm wide): €6,000 – €12,000
Gable Dormer
A dormer with a pitched roof. Two sloping surfaces meet at a ridge. Classic appearance but technically more complex.
Prefab cost (200 cm wide): €7,000 – €14,000
Planning Permission: When Is It Permitted Development?
Permitted Development (Rear of Property)
A dormer at the rear is often permitted development if:
| Condition | Requirement |
|---|---|
| Location | Rear roof slope (not front or street-facing) |
| Height | Maximum 1.75 metres (measured from roof plane) |
| Width | Maximum two-thirds of the roof width |
| Distance from ridge | Minimum 0.5 metres below the ridge |
| Distance from eaves | Minimum 0.5 metres above the gutter |
Always check with your local planning authority before starting.
Materials List: Flat-Roof Dormer 200 cm Wide
| Material | Quantity | Approx. Cost |
|---|---|---|
| Prefab dormer (frame + glazing + sides) | 1 unit | €3,500 – €8,000 |
| Structural beam (steel RSJ or LVL timber) | 1 piece, 250 cm | €200 – €600 |
| Rafters and roof sheathing | 5 m² | €80 – €150 |
| EPDM flat roof membrane | 3 m² | €60 – €120 |
| Lead flashing or zinc trim | 4 m | €120 – €300 |
| Roof tiles for reinstatement | 15–25 tiles | €50 – €150 |
| Insulation (PIR boards, 80–100 mm) | 3 m² roof + 6 m² sides | €150 – €350 |
| OSB board or plasterboard | 5 sheets | €60 – €120 |
| Sealant and fixings | Assortment | €50 – €100 |
Dormer Insulation: 2026 Standards
| Element | Minimum U-value | Recommended |
|---|---|---|
| Dormer roof (flat) | U 0.18 W/m²K | U 0.13 W/m²K |
| Dormer sides | U 0.18 W/m²K | U 0.13 W/m²K |
| Glazing | U ≤ 1.4 W/m²K | Triple glazing (U ≤ 0.8 W/m²K) |
Step-by-Step: Install a Dormer in 10 Steps
Step 1: Preparation and Planning
- Confirm exact dimensions and order the prefab dormer.
- Commission a structural engineer for the beam calculation.
- Book a crane for lifting the dormer into position.
- Arrange scaffolding for safe working at height.
Step 2: Prepare the Interior
- Mark the opening from inside.
- Install temporary props to support the roof structure.
- Remove insulation and inner lining in the marked area.
Step 3: Remove Roof Tiles
- Remove tiles in the working area plus 60 cm all around.
- Remove flashings, gutter and any lead work.
- Store tiles for reuse at the junctions.
Step 4: Cut Through the Roof Sheathing
- Saw through the sheathing on the marked lines. Do not saw through the rafters.
- Remove sheathing from the opening area.
- Remove rafters in the opening, propping the structure first.
Step 5: Install the Structural Beam
- Install the beam over the opening.
- Bear the beam on rafters on each side.
- Structural calculation is essential – not optional.
Step 6: Fix the Studs and Sill
- Mount the side studs.
- Mount the sill plate.
- Check plumb and level throughout.
Step 7: Crane In and Position the Dormer
- Attach lifting straps to the prefab dormer.
- Lift the dormer and lower onto the prepared opening.
- Two people on the roof guide it into position.
- Fix the dormer to the studs and sill.
Step 8: Weatherproof the Junction
- Lay EPDM or bitumen on the flat roof of the dormer.
- Fit lead flashing or zinc trim all round.
- Re-lay tiles around the dormer working from bottom to top.
- Seal all junctions. Test with a garden hose.
Step 9: Insulate and Line the Interior
- Fix PIR insulation boards inside.
- Fit vapour control layer.
- Fix plasterboard.
- Skim and decorate.
Step 10: Finish the Frame and Windows
- Apply external sealant around the frame.
- Fit interior window board and reveals.
Common Mistakes
| Mistake | Consequence | Solution |
|---|---|---|
| No structural calculation | Unsafe roof structure | Always use a structural engineer |
| Poor lead flashing | Leaks around the dormer | Have lead flashing done by a roofer |
| No vapour control layer | Condensation, mould | Fit VCL fully and tape all laps |
| Working on the roof without scaffolding | Fatal fall risk | Always use properly fixed scaffolding |
FAQs
What does a dormer cost?
DIY (prefab + materials + crane) costs €5,000 to €12,000. Contractor €10,000 to €25,000.
Do I need planning permission?
Not always. Rear dormers are often permitted development if the height is under 1.75 m, width under two-thirds of the roof and minimum 0.5 m from ridge, eaves and sides. Always check with your local authority.
How long does installation take?
With a prefab and a crane, the dormer itself goes in one day. Preparation 1–2 days. Interior finish 2–4 days. Total 5–8 working days.
Building Plans and Guides
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