With the average extension cost split between 10-20% fees, and the remaining 80-90% split fairly evenly between the external shell and the interior fittings, choosing a shell only extension can provide a huge cost saving!

What is a shell only extension?

An extension shell includes the external build of your extension, without any of the internal fit-out included. This means that contracting out the shell includes the construction of the structure and that this stage can be completed much more quickly than a full extension.

The scope of a shell extension will vary, so it is important to have a clear understanding of what this includes. For example, some extensions will require interior structural work, such as fitting lintels or RSJs, in addition to ensuring that robust foundations, walls and roofing are in place.

Usually, a shell will be required to be watertight, and have provision made for piping and utilities to be added at a later stage.

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Why do people choose shell only extensions?

There are lots of factors behind the decision, some include:

  • Having the necessary skills to carry out the internal work
  • Saving money on contractors fees by carrying out work yourself
  • Wishing to have more control over the internal fit-out
  • Wanting to use specific contractors for the internal work
  • Having a limited budget, and wishing to construct your extension in stages
  • Having plenty of time to commit to the internal decorating

Factors that affect the cost of a shell only extension

As with any extension, the size, complexity and shape of your property will dictate whether your extension costs are at the upper or lower ends of the typical price spectrum.

Factors to consider include:

Bespoke or off the shelf
Many extension shells are pre-fabricated, in which case you can purchase your shell and have it delivered and erected exactly as shown in the display specification. In this case, you will know in advance the exact cost of the shell you have chosen.

Size of your extension
A shell extension is a cost-effective option if you have the time and skills to carry out the internal work yourself. However, the cost of your extension will vary considerably depending on the size.

For example, a double-height construction will cost more than a single storey, and a complex layout will be more costly than a simple square or rectangular shell.

Raw materials
Usually, you would expect to negotiate a price for the shell construction as an inclusive project. The raw materials you choose for your build will impact this cost. The basic build will include laying foundations, damp coursing, building blocks, bricks, rendering – choosing the best materials within your budget will mean that your extension is sturdier and will require much less maintenance in the future.

Another way to reduce costs is to consider whether it would be cheaper to provide the raw materials yourself. Some homeowners choose to buy their materials directly through a trade account and thus pay their contractor purely for the labour element of the shell.

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Shell only vs. full extension - what is the difference?

A full extension includes delivering a completed extension, ready to use and to live in. A shell only delivers only the external element of the extension, so you will be left with a watertight space, laid on solid foundations.

The additional work required to complete your extension will depend on exactly which works were included with the shell, but typically will include:

  • Erecting internal walls – stud work, plasterboard, plastering
  • Utilities – running pipework for electricity, water and gas as required
  • Glazing & doors – depending on whether frames and glazing were included within your shell
  • Internal carpentry – such as batons, doors, skirting
  • Flooring – typically your shell will include the floor up to the concrete being laid

What are the benefits of a shell only conversion?

A shell only conversion is the middle ground between a self-build and a fully contracted project. The benefits of choosing this route are:

  • Being able to control and manage every element of the internal fit-out
  • Working to your own schedule, and managing the internal fittings at your own pace
  • Having full oversight of every aspect of the internal build
  • Savings costs on professional labour for work you can complete yourself
  • Being able to budget strategically, perhaps having the shell erected and then completing the project over time as finances allow
  • Using your own contractors for each aspect of the internal fit-out
  • Having autonomy over the process, rather than relying on one contractor to deliver a completed extension

If you have a garage and are thinking of applying a shell only extension to add a second story extension to your home, find out the cost of extending over a garage.

As with any extension, the size, complexity and shape of your property will dictate whether your extension costs are at the upper or lower ends of the typical price spectrum.

Frequently asked questions

How long does a shell only extension take?

The time taken to construct a shell extension will depend on whether you have an off the shelf shell, or a custom build project.

Typically, a full extension takes between three and six months to construct. The complete process including the design and applications for any permission may take around one year.

The external element of the extension accounts for anywhere between 30-50% of the work, depending on the complexity of the design and scope of the contract, so you could reasonably expect your shell to be completed in around three months from the start of the build.

A larger or very complex project might, of course, take longer!

Do I need planning permission?

Lots of extensions do not require planning permission. If the extension is fairly small, and does not exceed any of the construction borders of your existing property it is often possible to build under permitted development (PD).

For example, if your extension is to the rear of your property, does not extend out more than 3 metres for a terraced or semi-detached home, or more than 4 metres for a detached home, you might not need permission.

It is always best to check with your local planning authority before beginning any work and to ask for confirmation as to whether you are exempt from needing permission in writing.

Remember that listed buildings will always require Listed Building Consent before any modifications or building work can take place.

What are the stages/process of a shell only extension?

The construction of a shell extension follows the same process as for a full extension, albeit ending once the external structure has been completed.

Stages typically work as follows:

Stage 1: Planning
The site is prepared for work, and any clearance takes place. Raw materials are delivered to site, and any construction equipment and machinery arrive.

Stage 2: Groundwork
The foundations are dug, and any reinforcements are laid. Pipework, draining and service cables are laid within the foundations, and the concrete footing is then poured and levelled. This will usually require approval from Building Control to ensure it is safe.

Stage 3: Superstructure
The brickwork arrives, with damp proofing carried out, any concrete lintels inserted, and the insulation fitted. The concrete slab for flooring is poured and again usually checked by Building Control.

Stage 4: External Walls
The lintels, door frames and window frames are then fitted, along with block work and brickwork carried out. Cavity wall insulation is fitted, along with any wall ties where new walls need to be fixed to existing walls. Window and door lintels are fitted.

Each of these stages depends on your agreement with your contractor; for example, whether door and window frames are included within the shell. If glazing is not included, then typically the contractor will fit watertight covers to the window and doorframes to keep the building protected from the elements until the glazing is added.

What are the pitfalls to look out for?

Good planning is essential to a project that involves different contractors, or where you plan to carry out any elements of the work yourself.

Make sure that both you and your builder are clear on what is included within the shell contract, that the deliverables are agreed and that you have clear timescales in place to work to.

A common mistake is to forget about the entry points for your utility services, as these should be considered when the initial foundations are laid. Service ducts should be included within the shell unless you have plans to fit these later.

Another important aspect is to think about the fit-out, and how this will work along with the shell construction. For example, if you plan to use a separate contractor for the internal fit-out, there will be crossovers in ensuring that factors such as the ceiling height are clear from both perspectives; so that the shell is constructed to the finished dimensions, and not to structural dimensions.

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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