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Dormer Windows: How to Get the Design Right

Siberian larch and slate tile cladding on renovated bungalow
(Image credit: Simon Maxwell)

Designing dormer windows for an existing house can be tricky. Although ideal for creating ‘rooms in the roof’, the pitch, roof line and how it will interact with existing the house and those around it will need to be carefully considered.

When things go wrong with the design, they can ruin the first impression and kerb appeal of a property as it impacts the overall design so talk to an architect or designer when planning in some additional head-height.

Terrace houses are the ideal candidates for dormer windows

Terrace houses are the ideal candidates for dormer windows; this project chose to use a similar-coloured cladding material to the roof (Image credit: IQ Glass)

How Much Does It Cost to Add a Dormer Window?

A dormer loft conversion in the UK can cost anything upwards of £20,000 or around £500 – £600/m². The cost for a master bedroom plus en suite will naturally sit at the higher end of the spectrum at around £35,000 – £45,000.

This is likely to be a good investment no matter the property as Nationwide says a master suite conversion can add a value of around 20% to the value of a three-bedroom, one-bathroom house (although beware of area price ceilings). As the Office for National Statistics found the average house price in June 2019 to be £230,292, this could equate to around £46,000.

The inside of this terrace house is full of natural light and has a comfortable head-height thanks to the loft conversion and dormer extension

The inside of this terrace house is full of natural light and has a comfortable head-height thanks to the loft conversion and dormer extension (Image credit: Chris Snook)

Do I Need Planning Permission to Add a Dormer Window?

Planning permission is not required in most cases, as long as the dormer window design does not exceed the highest part of the roof, among other specific parameters. However, it is always best practice to check with your local planning office that the work does fall within your permitted development rights before any works begin.  

If the house is in a conversation area or the materials chosen for the dormer’s exterior contrast in style to the existing house you will need building consent for any significant works.

What is a Dormer-style Window?

From the French ‘dormir’ meaning ‘to sleep’, dormer windows are vertical units within a roof of their own, positioned, at least in part, within the slope of the roof. Ideal for those who can’t extend sideways or to the rear in urban areas or for self-builds with height restrictions, dormer windows can come in all shapes and sizes depending on what is right for the house. 

One-and-a-half-storey self build with dormer windows

The dormer windows included in this one-and-a-half storey self build match the large gable and follow the roof line design (Image credit: Simon Maxwell)

What Types of Dormer Window Can I Get?

A gable fronted dormer is the most common type of dormer window and traditionally has a simple pitched roof sloping to two sides. A vertical frame supports the planes to form a triangular section below the roofline.

A shed dormer has a sloped single flat plane roof in the same direction of the roofline but at a shallower angle.

This restored cottage features shed dormer windows

This restored cottage features shed dormer windows (Image credit: Darren Chung)

A hip roof dormer slopes on three planes and converges at one point.

With a curved roof and no sides, the eye brow, or eyelid dormer style gradually emerges as the roof moves up and over the dormer in a flattened bell curve.

Designing the Best Dormer Window 

Many dormers in early cottages are not original – a fact many self-builders forget – as there were no window openings at the first floor. So vast numbers of cottage dormers are actually retrofits — and yet so often they appear original. Why? Because they are not too big, and they are in proportion with the roof, which is both large and steeply pitched.

Many modern self-builders end up with one-and-a-half storey house with dormers as a result of planning restraints. The lesson here is still the same: keep the dormer windows in proportion.

Siberian larch and slate tile cladding on renovated bungalow

The homeowners of this dated bungalow transformed it with a radical remodel and dormer extension scheme; it cost £160k and added 78m² (Image credit: Simon Maxwell)

Generally the most unsatisfying visual feature of spec-built ‘dormer bungalows’ is usually that the roof windows are too big. The front elevation is out of proportion as a result.

Traditional cottages are perhaps the ‘easiest’ place in which to position a dormer — that is, the situation in which you are least likely to get it wrong. Keeping the ridges of a dormer well below the main ridge will inevitably look better, whatever the style of the house.

It is best to place them slightly lower than halfway down the roof. If they threaten to get close to the ridge, then the usual practice is to flatten their roofs.

An elegant, zinc roofed contemporary custom build home in Sussex that is packed full of eco features

The roofline of this eco self build has a low roofline which makes the flat roof of the dormer window integrate seamlessly (Image credit: Alistair Nicholls)

Dormer Windows and Roof Pitch

There is no harm in having the roof pitch of the dormer steeper than that of the main roof. However, if they are kept the same it will usually be possible to have tiled – as opposed to lead – valleys.

It is also wise to allow the roof of the dormer to follow the style of the main roof (i.e. hips with hips and gables with gables). A dormer placed in a hipped gable should also have a hip, otherwise it will look very odd. With a shallow roof pitch, often clad with slates, a flat-roofed dormer will look much better than one with a pitched roof.

Above all, try to put in a great deal of effort at the design stage. Play around with the elevations until they look ‘right’. Then choose materials very carefully, so they do not make the dormer too bulky.

Avoiding Dormer Window Design Mistakes

Dormers need to be an integral part of the overall house design. Designs should be generally subservient to, or at least picking up the design guidelines of, the existing roof shape. The tendency to go too large, particularly on smaller homes, is what leads to common mistakes.

With a large late-Victorian or Edwardian house containing a loft conversion, inserting roof dormers can actually enhance the elevation (especially in an urban situation) by adding to the verticality and reducing the ‘squatness’.

The design of this dormer window is balanced by the large rear extension

The design of this dormer window is balanced by the large rear extension (Image credit: IQ Glass)

With younger houses, however, the reverse can often be the case. The classic is the dormer in the roof at the front of a semi-detached pair. A dormer on one side may well create a look of imbalance and be a visual disaster. Of course this is assuming planning permission is granted for such a design. 

  • Avoid adding feature without any design confidence or consideration — also known as ‘stick-on features’
  • Make them an integral part of the design. (Try to find a style that suits the house, rather than simply the largest you can get away with)
  • Over-large or ‘boxy’ dormers will result in an ugly finish as they will overpower the rest of the house
  • Don’t over-insulate. Packing modern high performance insulation into the cheeks of a dormer will suffice without unduly increasing the thickness.