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 house and those around it will need to be carefully considered.

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.

However, when things go wrong with the design, 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.

How Much Does It Cost to Put In a Dormer and Do They Add Value?

bedroom in dormer loft conversion

This room in the roof in a listed house by Design Storey includes a bedroom with dressing area (image: Simon Maxwell)

A dormer loft conversion in the UK can cost anything upwards of £20,000 or around £500 – £600/m². A master bedroom plus en suite costing 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.

(MORE: Loft Conversion: Costs, Planning Rules and Fitting Out)

Do I Need Planning Permission to Install 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 before any works begin.  

dormer window in new cottage

Oakwrights and designer Pete Tonks created this self-build cottage with two small dormer windows that mirror the roof pitch and shape of the glazed gabel

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 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.

base architecture house with dormer windows

Base Architecture have designed this self build with two gabled dormer windows (one big, one smaller) to match the pitch of the gable wall to the far side of the house (image: Carl Blank)


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 

How Big Should Dormer Windows Be?

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.

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.

modern cottage with dormer windows

The dormers to this modern cottage by Border Oak follow the style of the main roof. The main roofline slightly imposes on the bottom of the dormer wall to ensure the design feels united 

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
remodelled bungalow with dormer widows

This dormer bungalow was dramatically remodelled and extended to fix a poorly designed flat-roof dormer and added two more in styles that match the house’s original pitch

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