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7 Brilliant Roof Extension Ideas

Loft Conversions

Perhaps like me, you’ve decided your house is lovely but would be even better with a little more space. Perhaps you’d like some brilliant roof extension ideas to help your new design stand out from the crowd?

To help visualise your options, our architects have designed nine different 3D models of different roof types and shapes.

Although an architect’s design can look fantastic on paper, it’s vitally important that it obtains planning permission and falls within your budget. We have also helped with that too.

Under each 3D image we’ve discussed:

  • Planning permission vs permitted development
  • The kind of space different types of roofs create
  • Construction costs

Table of Contents

1. Loft Conversion: Roof extension ideas

1.1. Hip to gable: roof extension

3D model of a hip to gable loft extension

Planning Permission vs Permitted Development

A hip to gable roof extension is one of the most popular. This is because it adds a huge amount of usable space to your existing loft. In many cases, this type of roof extension tends to be built via the permitted development route i.e. work that can be carried out to a house without requiring planning permission. If you live in a conservation area or a flat, it is doubtful that you would be granted permitted development rights as the local planning authority would not grant permission. The reason for the refusal would be the negative effect upon the area’s character due to the increase in size. However, if you own a house then this is a great way to add that much-needed space.


This type of extension only applies when the side of your house benefits from a hipped/slanted roof. Rather than remaining slanted at 45 degrees, the hip raises vertically, forming a gable or flat wall. This creates a huge amount of usable space.

Construction Cost

For an average size semi-detached house, the cost of both a hip to gable and full-width rear dormer loft extension would be between £50,000 and £60,000 (£1,700 and £2,300 per square metre).

1.2. Rear dormer window: roof extension

3D model of a full width rear dormer window

Planning Permission vs Permitted Development

Similar rules would apply to both full-width rear dormer windows and hip to gable loft extensions. If your house isn’t listed, nor in a conservation area, permitted development would usually be the best route.

This is because planning policy, (which can be restrictive), won’t apply when using permitted development. One of the main factors determining a permitted development rear dormer window size is the increase in volume. The following limits apply:

Terraced House: 40 cubic metres

Semi-detached or detached house: 50 cubic metres

On a typical semi-detached house, 50 cubic metres is sufficient space to construct a hip to gable and a full-width rear dormer window whilst complying with the allowance.

If you live in a flat conversion or a conservation area, the planning policies for dormer windows tend to disallow such big structures. In this case, the dormer window would need to be smaller and a more subordinate feature of the roof. This is discussed in more detail below.


A full-width rear dormer window can create a huge amount of space. This is because most pitched roof houses have very limited head height unless measured when standing below the ridge (the highest point). The roof pitch often descends at about 45 degrees meaning there may be a lot of floor area but not sufficient head height. The dormer windows increase the floor to ceiling head height, allowing you to take advantage of far more space. When coupled with a hip to gable extension, most semi-detached houses can accommodate a master bedroom and ensuite bathroom.

Construction Cost

The construction cost of a hip to gable and rear dormer window would be approximately £55,000 – £70,000. However, if you own a terraced house, the cost of a full-width rear dormer window on its own would be approximately £30,000 – £40,000 (depending on the size).

1.3. Traditional Dormer Window: Roof Extension

3D model of a traditional rear dormer window

Planning Permission vs Permitted Development

The planning system treats traditional dormer windows slightly differently when compared to the two examples discussed above. Quite often, these types of windows are included where houses don’t benefit from permitted development rights. In such cases, local planning policies can differ slightly between boroughs. As a rule of thumb, planning policies state the dormer window should be half the width and depth of the roof slope (as in the example above). When it comes to conservation areas or listed buildings, planning officers might also ask for a more traditional design with a pitched roof.


Due to the relatively small size of these dormer windows compared to the size of the existing loft, the usable space is limited. The only increase in space is in the area directly under the dormer window. However, they provide a good way of bringing in more light and offering better views when compared to a simple Velux window.

Construction Cost

As the dormer window is small, it is more economical compared to the dormers discussed above. A traditional dormer window would cost approximately £10,000 – £20,000.

2. Single storey: Roof Extension Ideas

2.1. Flat roof rear extension

3D model of a flat roof single storey rear extension

Planning Permission vs Permitted Development

When it comes to single-storey rear extensions, the roof shape is less important when debating whether to choose permitted development or planning permission.

When it comes to permitted development, the rules concerning height say the eaves can be no higher than 3m with a maximum height of 4m. While local planning policies might differ slightly, flat roofs are acceptable in both cases.


In terms of space, the main advantage of a flat roof is that they are simple with clean straight lines, allowing for a sleek modern look. This is quite impactful on the inside of your house as the existing ceiling can connect seamlessly to the new one, giving a minimalist appearance.

Construction Cost

The exact cost of the work will depend hugely on the size, width and finish of the extension. However, in terms of the materials and labour costs of a flat roof, roof membranes are applied quickly and are less expensive when compared with some of the more complex roof structures (discussed in more detail below).

2.2. Dual Pitch: Roof Extension Idea

3D model of a gabled roof single storey rear extension

Planning Permission vs Permitted Development

For those of you who would like something, a little different and modern, the dual pitch roof might be the answer. They can often fall within both permitted development and the requirements of local planning policies.


The advantage of this kind of roof is that the ridge running down the middle creates a far higher floor to ceiling height when compared to a flat roof. This can make the room within it appear very bright and airy, creating a great sense of openness.

Construction Cost

When compared to a flat roof, a dual pitched roof would take slightly longer to construct due to the increased complexity of design. In addition, if you would want fancier or more modern glazing at the rear, costs may increase.

2.3. Crown Roof: Rear Extension

3D model of a crown roof single storey rear extension

Planning Permission vs Permitted Development

A more traditional pitched roof design can be obtained through both permitted development and planning permission. When working in conservation areas or with listed buildings, planners prefer this type of roof due to the more traditional design.


The pitched roof design offers increased floor to ceiling heights for the living space below, creating an enhanced sense of space.

Construction Cost

These types of roof are slightly more complex than the simple flat roof designs. They also take longer to build as a frame is erected and clay hung tiles attached. But the difference between this roof and a flat roof would be negligible in terms of construction cost.

2.4. Modern pitch: Rear Extension idea

3D model of a modern single storey rear extension

Planning Permission vs Permitted Development

Due to the varying ridge and eaves heights of this type of modern extension, they can be more complicated to design and build. As permitted development guidance is quite rigid, these can be challenging to design hence it is sometimes easier to apply for planning permission as it offers more flexibility to be creative.

These roofs are best suited to areas with a mixed character, such as streets where houses benefit from lots of different types of historic extensions. In these areas, something a little out of the ordinary wouldn’t look out of place, as no two houses look the same.

There are also instances for uses within conservation areas and listed buildings as the juxtaposition between modern and traditional is an established part of architecture.


Given the irregular design, the spaces created can be quite unusual with varying head heights, giving a modern interior design approach.

Construction Cost

These are by far the most expensive types of extensions to build. Due to the more complex design and unorthodox roof structure, they take longer to construct than the more traditional roofs described above. The materials in these designs are also quite expensive compared to brick or clay hung tiles.


Although we have designed nine examples above, in reality there are an infinite number of possible designs. Hopefully we have given you a few ideas which can be the springboard for something really creative that stands out.

If you’d like to discuss any of your ideas (or hear some of our own), please feel free to call a member of our team for a free consultation at any time.

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