Dormer windows are a useful design device that provides additional head-height in otherwise cramped spaces. They are ideal for loft conversions or creating ‘rooms in the roof’ when building a one and a half storey home.

From the French ‘dormir’ meaning ‘to sleep’, dormers are vertical windows within a roof of their own, positioned, at least in part, within the slope of the roof.

When carefully considered, they can look great, but when things go wrong that can ruin the first impression and impact of your house design.

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 planning permission 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.

Adding Dormer Windows to Your Design

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.

Translate this to a modern one-and-a-half storey house – many self-builders end up with them because they cannot gain permission for two storeys – and the lesson is always the same: keep the dormer windows in proportion.

cottage style house with dormer window balcony

This one-and-a-half-storey home features bifolding doors and a balcony

Positioning Your Dormer Windows

If you look at any number of spec-built ‘dormer bungalows’, the most unsatisfying visual feature is usually that the roof windows are too big, meaning that the front elevation is out of proportion.

In many ways, the ‘easiest’ place in which to position a dormer – that is, the situation in which you are least likely to get it wrong – is in a traditional cottage. 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, but 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 and 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 – assuming you can get planning permission – may well create a look of imbalance, and be a visual disaster.

  • 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 imply the largest roof windows 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
american style house with dormer windows

This self build was inspired by traditional Cape Cod homes with well-proportioned dormers that complement the design on the whole

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