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 loft conversions and creating extra ‘rooms in the roof’, dormer windows are a great design device when carefully considered.

However, when things go wrong they can ruin the first impression of a house and impact the overall house design so talk to an architect or designer when planning in some additional head-height.

Do I Need Planning Permission to Add Dormer Windows?

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 before any works begin.  

traditional dormer windows in cottage

This oak frame eco home utilises a large dormer window to create added space in living areas

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.

Types of Dormer Window


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.

arts and crafts house with dormer windows

The homeowners of the Arts & Crafts-style self build chose to use shed dormers 


A hip roof – hipped – dormer slope on three planes and converging 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 a 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.

remodelled house with new dormer windows

This remodelled bungalow features new dormer windows which match the new New England-style

Positioning Your Dormer Windows

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.

bathroom with dormer window interior

The small original dormer of this 16th century cottage are in proportion with the rest of the building’s glazing

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.

thatched roof cottage with dormer windows

The thatched roof of this charming self build means the dormers blend seamlessly into the entire house design

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’.

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
bedroom in house with dormer window

This new self build was designed to mimic the traditional styles of the Lake District

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