What is a semi detached extension?

There are several ways to create a semi detached extension. Which is right for your property will depend on the layout of your home, where you have external space available to build on and your budget.

A side return extension creates a new living space along the side of your home. This is a popular choice with semi detached homes since they often have an unused strip of land running down one side.

Wrap around extensions utilise the side return space, in addition to a rear extension. This creates a larger ‘L’ shaped extension wrapping around the rear of your property.

A rear extension builds outwards towards the back of your home into the garden space. This type of extension could be either single or double-storey depending on your extension plans.

What other types of extension can be built on a semi detached home?

Other types of semi detached extensions include:

  • Dormer extensions to elevate the attic area and make this a usable space
  • Sunroom and conservatory extensions to any aspect of your house
  • Porch extensions to the front of your property
  • Garage extensions, usually to the side

What are the benefits of a semi detached extension?

Creating an extension is a cost-effective way to increase the floor space inside your home, build additional rooms or expand existing ones and accommodate growing space requirements whilst also adding value to your home.

Depending on your intended use of your extension a ground floor conversion can be used to increase the size of your kitchen or living space, build a dining area, or create a new room for a playroom, home office or utility area.

One of the most common reasons for choosing to build an extension is needing extra capacity for a growing family or change of home use. This is often a much lower cost option than moving home, and at the same time increases the value of your property should you come to sell in the future.

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How much value will a semi detached extension add to my home?

The average semi detached extension can increase your property value by:

  • 20% for a double-storey extension with a bedroom and ensuite
  • 12% per additional bedroom created
  • 6% per additional bathroom created

An extension of 25m2 will add around £60,000 to the value of a typical property, rising to an increase in value of closer to £170,000 in London.

What are semi detached extensions used for?

Extensions are versatile and tailored to your property and your requirements, so how you use the additional space is up to you and what best suits your family!

Many homeowners choose to use their semi detached extensions for the below uses.

Ground floor extension:

  • Expanding the kitchen or living room.
  • Creating a new playroom.
  • Building a conservatory, garden room or sunroom.
  • Adding a lean-to as a utility or storage space.

Double-height extension:

  • Creating any of the above ground floor uses, plus an additional bedroom or bedrooms, ensuites or bathrooms.

Box dormer extension:

  • Elevates the eave space in your attic.
  • Attic conversions can be used to create an additional bedroom, storage space, bathroom or home office.

There are no limitations on how you use your extension, aside from the budget you have to allocate to the conversion project. It is important to bear in mind what you will use your extension for, as this will impact the design and cost for factors such as insulation, electricity, fixed units and security.

How much does a semi detached cost?

Building an extension does not have a fixed cost since the design and size will contribute to the budget. Your choice of materials and the ease of access to the site will also affect the price, since bespoke or handmade materials will cost significantly more, and prefabricated panels will reduce the cost and build time.

The typical extension costs from £1,500 to £2,300 per m2, with properties in London and the southeast tending to be towards the higher end of the spectrum.

Depending on what size and location of extension you choose, it can be worth considering the return on investment. If you wish to avoid the planning application process, a simple ground floor extension is the quickest and most convenient choice.

For the optimal return on investment, a double-storey extension is more cost-effective per m2 since the groundwork and foundations will cost the same whether you are building one or two storeys.

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What are the important factors to consider when designing a semi detached extension?

With so many factors to consider, it is worth taking plenty of time to think about your extension plans. You can research other property conversions for inspiration, chat to local contractors and architects for design ideas and think about how much budget you have to allocate to the project.

If you plan to live on-site during the building works, be aware that for larger or more complex extensions there is likely to be significant disruption during the key building works, so it is wise to plan for this.

Other considerations include:

How much space you have to build on.
It can be exciting to think about a big expansion to your living space, but be aware that if you are building into your garden you will also be reducing your outdoor space. Think about other options if available such as utilising a side return to retain as much garden space as possible.

Whether you will need planning permission.
The cost of an application for permission to create an extension is £206, so this isn’t particularly expensive. However, planning can take several weeks and even months if any parts of the design are rejected, and incur other costs such as architects fees and surveyors costs.

What you will use your extension for.
Extending an existing room will include internal works to knock through external walls, so you will need to budget for the decorating work as well as the extension shell itself. New kitchens and bathrooms need a much larger budget given the costs of worktops, appliances and units, whereas simple standalone rooms cost much less.

For the optimal return on investment, a double-storey extension is more cost-effective per m2 since the groundwork and foundations will cost the same whether you are building one or two storeys.

Frequently asked questions

Do I need planning permission for a small extension?

Not necessarily; you do not need planning permission if your extension falls within the restrictions of Permitted Development (PD). PD means that you do not need to make a formal application for permission before you build. However, it is always worth checking with your local planning authority to confirm that PD does apply to avoid any future problems.

Extensions that are single storey may be up to half of the width of your existing property, and up to 3 metres long for a semi detached home. Most double-storey extensions do require planning permission, but for some designs, PD is permitted provided they are no longer than 3 metres.

To be classified as an extension for planning purposes, a large rear extension can be up to 6 metres for a semi detached home. However, this size of extension will require a prior notice to be submitted to the planning authority before work begins.

If you are considering adding a box dormer or hip-to-gable roof extension to the rear of your property, PD applies provided the new space created by your extension is no more than 40m3.

Whether or not you need planning permission, it is worth considering applying for a Lawful Development Certificate. This costs £103, is provided by the local planning authority, and acts as formal verification that your extension was built under PD.

This is particularly valuable to have if you come to sell your home and avoids any future queries.

How long does it take to build a semi detached extension?

Building an extension depends on many factors such as how much groundwork is required, whether any trees or shrubs need to be removed, and the size and materials of the extension itself.

An average extension takes around three months to build, although could be upwards of six months for a more complex build. Choosing extensions with prefabricated elements speeds up the build time since these are preassembled before delivery to site. Examples of prefabricated panels include garage extensions, lean-tos and conservatories. The shell of an extension can also be prefabricated in some scenarios.

The design and planning process is likely to take longer than the build, depending on whether you require permission in advance of the work. Should you be commissioning an architect, designer or surveyor it is wise to allocate around one year to the project from start to finish.

Can I build a new kitchen extension for my semi detached home?

Expanding or building a new kitchen is a common reason for choosing a semi detached extension. Kitchens can add significant value to your property, and are often knocked through to create open plan living spaces.

A bigger kitchen can be built within a semi detached property with a side return extension, a rear extension or as the ground floor of a two-storey extension build.

Side return extensions are very popular for semi detached properties since they avoid using up any garden space and utilise a usually redundant strip of land. Extending sideways makes use of this space and allows you to knock through the wall to build a larger ground floor footprint.

Many side return extensions incorporate skylights, Velux windows or light wells to bring more natural light into your ground floor.

What are rear extensions used for in semi detached conversions?

Rear extensions build backwards from the rear of your property and are less likely to require planning permission since they do not impact the front aspect of your home.

Properties with large gardens can accommodate a bigger rear extension, although a small extension can create a surprising amount of extra space.

Many rear extensions are used to connect the garden with the interior of your house and using Bifold or patio doors adds much more natural light. Glazing can be an expensive part of the extension budget, so shop around to get the best prices and compare the costs and energy efficiencies of different materials and double vs. triple glazing.

The uplift on price from double to triple glazed is usually around £100 per panel, although this can pay dividends in the long-term in reducing heating bills, increasing the energy efficiency of your home and providing cheaper options for insulation.

Standard size windows and frames are far less expensive than bespoke sizes made to order. If you are considering very large glazing panels, make sure to obtain at least three quotations before ordering their production.

How do I design a semi detached extension?

Some homeowners have a clear vision for how their extension should look. Others appoint a designer or architect to create a plan, and the aesthetic depends on your tastes and preferences.

Many extensions are created in sympathy with the existing property, with similar materials, colours and finishes so that the extension will look as though it has always been part of the original building.

Other extensions are used as a design statement or to modernise the property appearance. This can be a great way to provide a facelift to your home, making it stand out from other houses in the area and increasing its saleability. It is worth taking the time to consider how the extension design will fit in with your existing property if you are choosing a contrasting conversion.

Can a semi detached extension convert my home to open plan living?

It certainly can! Open plan living spaces continue to be very popular, and removing unnecessary internal walls opens up your home, provides a better flow of natural light and creates optimal floor space without the areas taken up by walls and doors.

Open plan living can be designed around your home and your way of living, but often consists of an open plan kitchen and dining area, or an open plan living space blending the lounge area with entertainment spaces.

How should I choose a contractor for my semi detached extension?

Building an extension can take a lot of time and budget, so it is essential to obtain at least three quotations to compare prices. Specialist contractors in the type of extension you choose will often be able to provide valuable advice about design, planning and materials, whereas a more general builder may be able to provide the practical work but not be able to input on how to create your ideal design.

If you are choosing specific materials or constructions, selecting a contractor with experience in that area is the best choice. For example, timber frame extensions, box dormer conversions or prefabricated conservatories are usually more cost-effective when sourced from a contractor with a niche in these areas.

Price is one consideration, but availability and quality of work are equally important. If you are keen for work to begin quickly then check with each contractor what his or her availability looks like before committing to a deposit.

Where possible it is always worth asking for references or testimonials; even better, if a contractor has completed projects in your local area you can go and take a look. This is a great way to evaluate their standard of work, gain inspiration for your build project and even – with permission – speak to previous clients to ask about their experience.

What should I look out for in my semi detached extension quote?

Quotations for extension work vary significantly, which is why it is so important to take your time when selecting the contractor you work with.

An end-to-end contract will include every aspect of the build including roofing, flooring, electrics, plumbing, decorating etc. Other works may include only the external construction, or exclude the cost of specific materials so you need to be clear about what is and is not included to be able to budget accurately for any additional costs.

If you plan on completing any part of the work yourself – for example, the interior painting and decorating – make this clear from the outset to avoid duplicating your costs.

Reputable contractors will always state on their quotations whether or not VAT is included and whether or not it is applicable. This is something to watch out for since a quote excluding VAT will increase in the total price by 20% when it comes to the point of payment.

Agreeing to payment schedules in advance is also essential to avoid any disagreements or misunderstandings. Most contractors will require a deposit upfront, and larger extension projects may have staged payments at set intervals of the build.

Make sure to include snagging in the contract or plan of work. Snagging is your opportunity to check every corner of your build carefully when the work is completed, including small details such as making sure doors close flush and all of the utility points work correctly. It is best to avoid paying the final instalment until any snagging works are completed.

Rich Crossley
Having spent 30 years working all over the world for top-tier investment banks, I’ve owned and developed houses all over the world – Europe, the US and Asia. I’m now based back in the UK and involved in the property industry – oh, and I’m a keen DIY enthusiast!
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