Making use of the space you're already paying for
Put that unused side return to work with a side extension.
Side Extensions
The side return that's been sitting idle
Side extension conversations tend to start with a specific frustration about wasted space, rather than a request for a particular type of build.
Homeowners tell us things like: “We’ve got this narrow strip down the side that does nothing,” “Our old garage is just storage now, may as well be a room,” “We want the kitchen to feel like it belongs to the whole width of the house.”
A side extension builds outwards into that unused space, often alongside or instead of an existing garage, to widen the kitchen, add a utility room or create the extra width needed for an open-plan layout. Many single-storey side extensions fall under permitted development, provided the design stays within set width and height limits, which can make this one of the more straightforward extension types to get moving.
We assess what your property allows, whether that’s permitted development or a planning application, and how to make the added width count.
What's included in our side extensions service
You’ll get the very best support from concept through to build.
Measured property survey
Precise recording of your side elevation, boundary distances and any existing garage or outbuilding being incorporated.
Side extension layout design
Concepts built around widening your existing rooms rather than just adding a narrow annex.
Permitted development check
We confirm whether your side extension falls within permitted development limits on width and height.
Planning application management
Where a full application is needed, we prepare and submit it on your behalf.
Building regulations drawings
Technical detail covering party walls, insulation and structural openings.
Structural engineering coordination
Support with beams and openings where the side extension connects into the existing structure.
Three steps to the home you’ve always wanted
We talk through what’s not working in your current layout and what you’d like the new space to do for you.
01: Explore ideas
We talk through what the extra width should give you, and what the plot realistically allows.
02: Design + planning
We develop the layout and handle permitted development confirmation or a full planning application.
03: Build-ready drawings
You receive compliant technical drawings, ready for builders to quote against.
How much
does a side extension cost?
Side extension costs depend largely on width, whether an existing garage is being incorporated, and the specification of the finish.
As a very broad guide:
- Smaller side infill extensions may start from around £25,000
- A typical side extension widening a kitchen often ranges between £40,000–£75,000
- Larger side extensions, particularly those replacing a garage entirely, can exceed £90,000
Incorporating an existing garage structure can sometimes reduce cost compared with building entirely new footings, though this depends on the garage’s construction and condition. A proper set of drawings is the only reliable way to turn that broad range into an accurate quote.
Sarah & James’ Dream Home, Essex
Planning permission & permitted development for side extensions
Side extensions have their own specific set of permitted development rules, separate from those covering rear extensions.
A single-storey side extension can often be built without planning permission provided it’s no more than half the width of the original house, doesn’t exceed 4 metres in height, and sits far enough from the road. Side extensions of more than one storey do not qualify for permitted development at all, and almost always require a full planning application, as does any side extension on a property within a conservation area.
Because the width limit is one of the most common reasons a side extension design gets scaled back, we check this early, so the layout is built around what’s actually achievable rather than redesigned later.
Advice
on planning permission and permitted development rules
Support
with planning applications
Guidance
tailored to your property and goals
Areas we cover
Essex and beyond
We regularly design side extensions for homeowners across Essex and the South East looking to make better use of underused space.
Our team supports side extension projects in Chelmsford, Southend, Basildon, Brentwood, Rochford and the surrounding towns.
Why Design Spec?
Our goal is to make your house transformation simpler. Clear costs and a design that earns its place.
Fixed-fee pricing
One agreed cost, regardless of how many revisions the layout goes through.
Unlimited design revisions
Time to get the kitchen-to-garden flow right before anything is built.
One-to-one project support
A dedicated contact who knows your project and your property.
What we design
We get your loft to work harder, whether it’s destined to become a new bedroom or home office.
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Garages have potential for so much more than storage. We can help you unlock its value.
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Ever looked at your house and thought it could be bigger and better? We can help you make it happen.
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Add more rooms, more space and more value to your home with a double-storey extension.
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They say the kitchen is the heartbeat of the home. We’ll work with you so it really lives up to that title.
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Let’s make better use of that garden and carve out some more living space at the back.
Find out moreFAQs
Not always. A single-storey side extension often falls under permitted development provided it’s no wider than half the original house, stays within height limits and isn’t sited on a property within a conservation area. Side extensions of more than one storey do not qualify for permitted development and will need a full planning application.
Even where permitted development applies, obtaining a Lawful Development Certificate confirms the build’s legal status in writing, which is worth having in place before construction starts.
Costs depend on the width being added, whether an existing garage is being absorbed into the build, and the finish specification. Smaller side infill extensions tend to start from around £25,000, while larger side extensions replacing a garage can exceed £90,000.
Getting an accurate figure means pricing against completed drawings rather than a generic per-square-metre estimate, since incorporating existing structures like garages can shift the cost significantly either way.
Under permitted development, a single-storey side extension generally cannot exceed half the width of the original house. This limit exists specifically to prevent side extensions from overwhelming the property’s original proportions when viewed from the street.
Because this is one of the more restrictive permitted development limits, it’s worth checking early in the design process, particularly if you’re hoping to create a wide, open-plan space that spans the full depth of the house.
Yes, and this is a common approach, particularly where the garage is used mostly for storage rather than parking. Incorporating an existing garage structure can sometimes reduce foundation costs compared with building an entirely new side extension, depending on the garage’s construction.
The design needs to account for the garage’s existing foundations and any structural differences between the old and new sections, which is something we assess as part of the initial survey.
A side extension builds outwards into the side return only. A wraparound extension combines that side infill with a rear extension to form a single L-shaped space, typically creating a much larger open-plan kitchen, dining and living area.
Because the side element of a wraparound extension is subject to the same width restrictions as a standalone side extension, the two project types often need to be planned together rather than treated as separate decisions.
It can, particularly on narrower plots where the side extension sits close to a shared boundary. This is one of the main considerations local authorities look at when a side extension does require a full planning application.
We factor boundary distance and potential overshadowing into the design from the outset, which reduces the risk of objections where a planning application is needed.
Builders typically need planning drawings, building regulations drawings and structural information covering how the new extension ties into the existing side wall, particularly where an old garage structure is involved.
Design Spec provides a complete, build-ready drawing package so builders are pricing against clear, consistent information rather than making assumptions about how the new and existing structures connect.