Single Storey Extension: A Straight-Talking Build Guide
What a rear extension really involves: the build stages, the planning rules, the cost drivers, and the design decisions that make it work.
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8-12
WEEKS ON SITE
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10%
SENSIBLE CONTINGENCY
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25+
YEARS OF BUILDING
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5
TOWNS COVERED
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Most families do not need a bigger house, they need a bigger kitchen and somewhere to put the wellies. A single storey extension solves exactly that, and June is when the projects that finish before Christmas get moving. Here is what the build actually involves, what decides the cost, and the questions worth asking before anyone digs a footing.
What a Single Storey Extension Involves, Stage by Stage
The sequence is the same on almost every job: design and approvals first, then groundworks and footings, the shell with walls and roof, followed by windows and doors to make it watertight. After that come first fix electrics and plumbing, plaster, second fix, and decoration.
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1
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Design and approvalsLayout, drawings, planning or permitted development checks, and structural calculations. |
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2
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Groundworks and footingsExcavation, foundations, and drainage runs sorted while the ground is open. |
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3
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The shellWalls up and roof on, with steelwork installed where the house opens into the new room. |
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4
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WatertightWindows, doors, and roof lights fitted so the weather stays outside. |
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5
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First fix and plasterElectrics and plumbing roughed in, then the walls and ceilings plastered. |
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6
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Second fix and decorationSockets, lights, flooring, kitchen or utility fit-out, and the final coat of paint. |
On site, a typical single storey rear extension runs eight to twelve weeks from digging to decorating, weather and complexity depending. The paperwork phase before it can take just as long, which is why summer enquiries become autumn completions rather than summer ones.
Services deserve early thought because they hide in exactly the ground being dug. Drains crossing the footprint may need diverting or building over with approval, the electrics in the new room need planning back to the consumer unit, and a downstairs loo or utility plumbing is far cheaper to rough in now than to retrofit later.
Good builders sequence the messy work to keep the main house liveable, and we covered the household side of that in our guide to preparing your home for major renovation work.
Planning Permission or Permitted Development
Plenty of single storey extensions need no planning application at all, because permitted development rights cover rear additions within set limits on depth, height, and coverage. The rules have detail in them, and the Planning Portal sets them out for every property type.
Building regulations apply either way, covering structure, drainage, insulation, and electrics, with sign-off through building regulations approval. A builder who handles building control as part of the job removes the most stressful paperwork from your side of the table.
Neighbours come into it on most semis and terraces, because footings within three metres of next door’s foundations bring the Party Wall Act into play. Served early, the notices never delay anything, and conservation areas or flats simply move the project onto the planning route rather than ruling it out.
What Actually Decides the Cost
Size matters less than people expect, because the expensive parts of an extension cluster in particular places: foundations near drains or trees, structural openings into the existing house, and the kitchen or bathroom fit-out inside the new space. A simple shape with a sensible roof is the budget’s best friend.
Opening the back of the house into the new room usually means steel, and the structural work is where experience shows. Our RSJ installations page explains how the supports go in and why building control wants calculations rather than guesswork.
Get a fixed-price quote with a clear specification, and treat any price that arrives without a site visit with suspicion. After 25 years across Widnes, Liverpool, Runcorn, Warrington, and St Helens, we have priced enough jobs to know where surprises hide.
Hold a sensible contingency of around ten per cent for the things no survey can see, mostly in the ground. If it goes unspent, it becomes the better flooring or the extra roof light, and if it is needed, the project absorbs the surprise without drama or difficult conversations.
Designing the Space So It Works
The best single storey extensions are designed from the inside out: where the table sits, where the sofa faces, where the light comes from in the evening. Roof lights over the kitchen run, wide glazed doors to the garden, and a utility corner hidden behind the everyday space turn a box into a room.
Think about the junction with the old house too. A knock-through that lines the new space up with the existing kitchen changes how the whole ground floor flows, and our property extensions team designs both sides of that opening together.
Storage is the detail families thank themselves for later. A bank of full-height cupboards along one wall, a window seat with drawers, or that hidden utility corner swallow the clutter that otherwise lands on the new island, and they cost least when they are framed into the build rather than bought afterwards.
Extension Mistakes That Cost Real Money
The most expensive mistakes happen before anyone digs. Changing the design mid-build is the classic one, because a moved doorway on paper costs nothing while a moved doorway in blockwork costs labour, materials, and programme. Settle the layout, the glazing, and the kitchen positions before the first delivery, and the budget behaves.
Comparing quotes on the headline figure alone is the second trap. A low figure that excludes drainage alterations, structural steel, or making good is not a cheaper extension, it is an unfinished one. Insist on a written specification with every quote, so the numbers describe the same job.
The third mistake is designing the extension in isolation. The new room changes how the hallway, kitchen, and garden doors work together, which is why we sketch the whole ground floor even on single-room projects. Families planning bigger changes often fold the extension into a full house renovation, where one programme covers everything.
Frequently Asked Questions
How much does a single storey extension cost in the North West?
Most fall within a broad band driven by size, groundworks, glazing, and what goes inside the room, with kitchens adding the most. Rather than quote a misleading average, we price from a site visit with a fixed written quotation, so you compare like for like. Free designs and quotes are part of the service.
Do I need planning permission for a single storey extension?
Often not. Permitted development rights cover many rear extensions within limits on depth, height, and how much of the garden is covered. Corner plots, flats, and conservation areas change the answer, so it should be checked early. Either way, building regulations approval is always required, and we handle that process for you.
How long does a single storey extension take to build?
Eight to twelve weeks on site is typical for a straightforward rear extension, from groundworks to decoration. Design, approvals, and structural calculations add weeks before the start date, which is why projects enquired about in June tend to complete in autumn. A clear programme from your builder keeps every stage visible.
Can we live in the house during the build?
Almost always, yes. The new room is built outside the existing wall, so the house stays weathertight until the knock-through, which is sequenced late and protected with dust screens. Expect some noise and disruption around groundworks and the opening-up, and plan a temporary kitchen arrangement for a short stretch.
Will an extension add value to my home?
Done well, a single storey extension that creates a kitchen diner or family room adds genuine market appeal, and in much of the North West the gain comfortably justifies the spend. The bigger return is daily: space that fits how your family actually lives. Bad design, not size, is what fails to pay back.
Do you handle building control and inspections?
Yes. We project-manage the approvals, book the inspections at each stage, and keep the paperwork tidy through to completion certificates. Structural calculations for any steels are produced by an engineer and submitted as part of the package, so nothing rests on guesswork and nothing is left for you to chase.
Do I need an architect for a single storey extension?
Not always. Many single storey extensions are designed perfectly well by an experienced builder working with a structural engineer, and we provide free designs as part of our quoting service. An architect earns their fee on complex sites, unusual designs, or sensitive planning situations. We will tell you honestly which camp your project sits in.
Can my extension include a downstairs toilet or utility room?
Yes, and it is the cheapest moment you will ever have to add one. The drainage, water, and electrics are already being designed, so a toilet or utility corner adds little to the groundworks while adding daily convenience and resale appeal. Tell your builder at design stage, because retrofitting either later costs far more.
Start the Conversation This Summer
If a bigger kitchen and a better back garden connection are on the family wish list, June is the month to start the conversation for an autumn build. Send over your idea, and Tommy and the team will tell you straight what is realistic, what it costs, and how to get it in the diary. Free designs and quotes, family-run, fully insured, and you can contact the team here to get started.
Ready to sketch the new kitchen?
Free designs and quotes from a family-run Widnes builder with over 25 years of hands-on experience.
