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Semi-detached House Extensions: A Comprehensive Guide


Did you know that a third of us live in semi-detached houses?

I didn’t either until I googled it.

That explains why semi-detached house extensions are so popular.

However, planning an extension can be daunting as there are so many different factors to consider.

To help give you some inspiration and answer some common questions we’re often asked, our architects and planners have prepared this useful guide.

We cover topics such as: planning permission vs permitted development, construction costs, new building materials, and we’ve included lots of design ideas.

Table of Contents

1. Do I need planning permission for a semi-detached house extension?

I wish there was an easy answer to this question! When it comes to picking the best planning route, there are several competing factors to consider. The one which suits you best will depend on your specific site circumstances.

To help you make the best decision, we have picked 4 of the most common types of semi-detached house extensions to talk about below. For each, our planning consultants assess whether the Planning Permission or Permitted Development route might be best.

1.1. Single storey rear: semi-detached house extension

Single storey rear extension
The typical proportions of a single storey rear extension

Permitted Development: Permitted Development Rights are certain types of extension you can add to your semi-detached house without applying for planning permission. Unlike planning policy, Permitted Development legislation is black and white. This means there is little room for interpretation. It sets out the rules and your semi-detached extension either meets them or it doesn’t (brutal, we know!). Let’s face it, most homeowners want the biggest extensions possible to create the most amount of new living space. In terms of depth, there are 2 different options when it comes to single storey rear extensions for semi-detached houses.

Permitted Development:        3m deep single storey rear extension

Prior Notification:                   3 – 6m deep single storey rear extension

Planning Permission: Determining how big your semi-detached extension can be by going down the planning permission route is more complicated. This is because every local planning authority will have their own design guidance. As a general rule of thumb however, most local authorities will grant single storey rear extensions on semi-detached houses between 3 – 3.5m in depth.

1.2. Single storey side: semi-detached house extension

Single storey side extension
The typical proportions of a single storey side extension

Permitted Development: When it comes to single storey side extensions to semi-detached houses, they should not extend beyond the rear or front of your house. Second, they should be less than half the width of your house. This means that if your house is 8m wide, the maximum width of your single storey side extension would be 4m. As most semi-detached houses have space to the side, this is always a great option to add space.

Planning Permission: In my experience, planning policies for single storey side extensions are similar to Permitted Development legislation. This means you could achieve a similar size extension with either route. However, if you’re looking to connect your side extension to a new porch, or have your side extension wrap around the rear of your house, then planning permission is the only route available. How your local authority planners assess your wraparound extension will be based on your specific site circumstances.

1.3. Loft Conversion: semi-detached house extension

Semi detached house extension loft conversion
This is what a Permitted Development loft extension to a semi-detached house looks like

Permitted Development: Having been a local authority planning officer for a long, long time, I can safely say that over 95% of loft conversions in the UK are built through Permitted Development. The reason why is because this route gives you far more space than could be achieved through going down the planning permission route. There is quite a bit of Permitted Development legislation when it comes to lofts, but the most important in terms of achieving the maximum amount of floor space is:

  • The maximum amount of space that can be added to a semi-detached house extension is 50 cubic metres
  • No part of the loft conversion can extend beyond the exterior wall of your original house
Semi detached house extension planning permission loft conversion
This is what a loft extension to a semi-detached house looks like if you apply for planning permission

Planning Permission: Here is some sad news. Planning policies when it comes to loft conversions are quite restrictive in terms of size! Every local authority will have their own design guidance, but as a rule of thumb, dormer windows should be half the depth and width of the roof slope. This means that if your house is 8m wide your dormer window might only be 4m wide. And, if the height of your roof is 4m, your dormer window can only be 2m high. This doesn’t add much space, which is why we would always advise going down the Permitted Development route first (assuming you meet all the relevant legislation).

2. Semi-detached house extension ideas that will stand out from the crowd

2.1. Glass

Using glass in your new semi-detached house extension can be a great way to create the wow factor. We love glass because it creates the perfect visual contrast between the original house and the extension you’re adding. Be wary about using too much glass in south facing extensions though, as they can get very hot in summer. Your architect will also have to consider the design carefully. This is because glass makes it more challenging to meet the relevant building regulation energy efficiency standards.

Semi detached glass extension
Flood your living room with natural light with a glass extension

2.2. Timber

Most extensions are made from brick, but timber cladding is becoming increasingly popular. We love it! We especially like black or charred timber which can add a striking and modern aesthetic look to any extension. The second advantage is that it’s a far more sustainable option when compared to brick, especially when the wood is locally sourced.

Timber cladding extension
Timber cladding extensionTimber cladding not only looks fantastic but is also a sustainable option

2.3. Zink

Zinc cladding is a type of metal cladding. It comes in many different forms and sheets can be formed into shingles, cassette panels and flat-lock panels to suit any type of building. Here are a few other advantages.

Design flexibility: It can be shaped into many forms, including curves and sharp angles

Great longevity: The lifespan of zinc cladding can be in excess of 60 years

Corrosion resistance: Zinc does not rust so it will look better for longer

Aesthetics: Its natural grey appearance means it can be mixed and matched with other popular materials such as white render or brick

Low maintenance: Zinc cladding can be cleaned with hosepipe and sponge very easily

If you’re looking to stand out from the crowd why, not try this stunning zinc cladding?

2.4. Render

Rendering a semi-detached house extension is largely chosen for its aesthetic value. It’s a simple texture, and the best part is it can be applied in any colour. This makes it a versatile choice for mixing and matching with other common building materials. There are also other advantages. It can be used to help protect walls from damp during those rainy months. There are also acrylic insulated renders which can make your house more thermally efficient. Saving on expensive heating bills is always a plus!

White render extension
This mixture of white render and timber cladding makes for a truly unique extension

2.5. Copper

The great part of copper cladding is that it looks as good on traditional houses as it does on something a little more contemporary. Aside from looking incredible, there are a number of other advantages to copper cladding. Its durability, resistance to wind, rain and temperature fluctuations, along with its corrosion-resistant properties are some of the advantages.

Copper cladding extension
Add a bit of character to your rear extension with this weathered copper cladding

3. How much does a semi-detached house extension cost?

There are lots of different factors affecting the cost:

  • How many storeys you’re going to build
  • The size and shape of the extension
  • The quality of the build: standard, good, excellent
  • The build route you’ll take — how involved in the project you’ll be
  • The amount of glazing you’d like
  • Whether the extension contains a kitchen or bathroom
  • What fixtures and fittings you would like

To help give a better idea of the different construction costs for a typical single storey semi-detached house extension, here is a breakdown.

Quality of finishExtension sizeLow cost (m2)High cost (m2)Average total cost
Basic finish20 m2£1,900/m2£2,100/m2£40,000
Standard finish20 m2£2,100/m2£2,400/m2£45,000
Premium finish20 m2£2,500/m2£3,000/m2£55,000

4. How much value will a semi-detached extension add to my house?

4.1. Loft conversion: semi-detached house extension

Average value added: Up to 20%

As the floorspace in your loft already exists, this is one of the most affordable ways to create more living space to your semi-detached house. If you can add a master bedroom, walk in wardrobe and ensuite bathroom, this will add the most value to your house.

4.2. Single-storey: semi-detached house extension

Average value added: 5-8%

Single storey extensions are mostly used to enlarge existing living rooms and kitchens. There seems to be a move towards more open plan living, so if you can take down any existing dividing walls, that will certainly add more value.

4.3. Double-storey: semi-detached house extension

Average value added: up to 12%

Double storey extensions are fantastic as the groundwork for the ground floor will already be in place. This means there is less additional work for your builder to complete the first floor.

4.4. Garage conversion: semi-detached house extension

Average value added: 10-20%

Garage conversions are hugely popular because the space already exists. In terms of overall construction costs, it costs far less to make your garage habitable compared to building an entirely new structure. Why not convert your garage into a cosy home office to get away from the hustle and bustle of the main house?


There are an infinite number of design possibilities when it comes to semi-detached house extensions, which is why our team of architect’s love working on them.

If you need any advice or would like to run your ideas past our team of creative architects, feel free to call us anytime.

We’re always here to help.

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