Dormer Windows Guide | Homebuilding & Renovating

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

Dormer windows can be a useful design device where additional headheight is required – for instance when converting an existing loft or creating ‘rooms in the roof’ when building a one and a half storey home.

When done well, they can look wonderful 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?

In most cases, as long as the dormer window design does not exceed the highest part of the roof, among other specific parameters, planning permission is not required, but it is always best practice to check with your local planning office before any works begin.  

However, 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

Gabled

The most common type of dormer window, a gable fronted dormer traditionally has a simple pitched roof sloping to two sides. The planes are supported by a vertical frame to form a triangular section below the roofline.

Shed

A single flat plane roof which is sloped in the same direction of the roofline, but at a shallower angle.

Hipped

A hip roof – hipped – dormer slope on three planes and converging at one point.

Eyebrow

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

What Style Dormer Window Should I Choose?

Many self-builders forget that when early cottages were built there were no dormers — because there were no window openings at first floor level. 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.

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. Whatever the style of house, however, dormers inevitably look better if their ridges are kept well down from the main ridge.

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.

The Impact of the 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.

What to Avoid When Adding Dormer Windows

One of the key things to avoid is ‘stick-on features’ — features that are added with a total lack of design confidence. For loft conversions, dormers must be an integral part of the overall design, rather than simply the largest roof windows you can get away with.

If they appear ‘boxy’ and over-large the result will be terrible because they will overpower the rest of the house. An additional danger nowadays is the desire of many builders to over-insulate. In fact, modern high performance insulation can quite easily be packed into the cheeks of a dormer without unduly increasing the thickness.

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.