How much does a house extension cost?
House extension costs in Essex vary depending on the size, design, and extension type. Understanding the average house extension cost helps you plan your budget and avoid unexpected expenses.
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Examples of house extension costs
Here are some examples of how much an house extension might cost in 2025. The prices are based on what our vetted builders charge.
Detached
Detached house
extension costs
Semi- detached
Semi-detached house
extension costs
Terrace
Terrace house
extension costs
Bungalow
Bungalow house
extension costs
Thinking about extending? Here’s why It makes sense
A house extension is the easiest way to gain extra space without the stress of moving. One of the most stressful things is moving house, so why bother when you can just build a home extension and gain the space that way. Many of Kirkwood’s customers in Essex, especially in Chelmsford, Brentwood and Rayleigh, choose to extend instead of moving.
Example costs:
All estimated House Extension Costs cover building work, plumbing, electrical work, glazing, external doors leaving a plaster finish ready for decorating.
Detached houses
Detached house extensions usually cost more because the houses are bigger, so the extensions tend to be bigger too. The survey takes longer, and the drawings cost a bit more as there’s more to measure and more detail to include.
Small Flat Roof
Size 4m x 3m
Estimated costs:
£26,995+vat
Large Flat Roof
Size 8m x 3m
Estimated costs:
£45,995+vat
Semi- detached houses
Semi-detached houses can sometimes use the party wall, which saves building a new wall and digging foundations on that side. But if the neighbour objects, you might need a party wall surveyor. The homeowner is responsible for the cost, which is usually around £1,200.
Pitched Roof
Size 9m x 3m
Estimated costs:
£49,995+vat
Lean-to Roof
Size 7m x 3m
Estimated costs:
£39,995+vat
Terrace houses
A terrace house extension can be a bit trickier. There’s usually less drive space for deliveries and often no side access, which means everything has to come through the house. This doesn’t stop the job from going ahead. It just takes a bit more time, which can push the cost up slightly.
Pitched Roof
Size 6m x 3.5m
Estimated costs:
£43,995+vat
Gabled Roof
Size 5m x 4m
Estimated costs:
£41,995+vat
Bungalows
Bungalows usually come with plenty of space for deliveries, skips and side access. There’s often extra work needed on the existing roof because the new extension roof will likely sit higher and need tying in. That’s not something you usually deal with on other house types.
Large extension
Size 9m x 4.5m
Estimated costs:
£71,875+vat
Medium extension
Size 4m x 3.5m
Estimated costs:
£29,995+vat
Real house extension costs
Want to see real extension costs for projects in Essex?
Chelmsford - New London Road
Single story pitched roof extension 4m x 9m - £64,995+vat
Brentwood – Shenfield Road
Single story pitched roof extension 6.5m x 8.2m - £69,995+vat
What Does a House Extension Actually Cost?
House extension costs vary a lot depending on the type of build, the finish you want, and how complex the design is. The average house extension cost in Essex starts around £2,000 per square metre, but that’s just a guide. Some builds in places like Basildon or Southend come in lower, while others can go much higher depending on access, layout or local builder rates.
If you’re asking how much does an extension cost, the truth is it depends. The cost of house extension work also includes design, approvals, and structural checks. That’s why Kirkwood always starts with build cost advice. Its important you know how much the extension will cost, before committing to plans.
Extension costs Essex-wide follow the same pattern, square metre pricing is useful, but the details make the difference. If you’re looking at the cost of extension UK figures online, take them with a pinch of salt. Local planning and design advice makes all the difference.
What type of extension are you planning?
House extension costs vary depending on the size, roof type, foundation type, internal finishing and the area. Small extensions often cost more per m2 than larger ones. Extension costs in Essex also depend on access, planning requirements and design. At Kirkwood, we design extensions to suit your budget. If you’ve got £50k to spend, we’ll create something that meets your needs and stays within budget.
single storey extension cost
The average single storey extension cost starts from around £40,000 in Essex, but can go much higher depending on the size, spec and foundation type. A basic rear extension with patio doors, standard bricks and no steelwork, will be cheaper than something with roof lights, aluminium bifolds and a new open plan kitchen. In Upminster, we’ve seen small extension cost per m2 figures hit £2,500 because of site access, foundation type and internal specs.
Double storey extension cost
Double storey extension costs usually offers better value per m2, but there’s more involved, structural work, steel, extra plumbing, design work. In places like Chingford, Billericay and Brentwood, the cost of building an extension over two floors can start around £65,000 and go up from there depending on size. We design with structure and budget in mind from day one.
Kitchen extension cost
Kitchen extension cost can vary a lot depending on the finish. A rear kitchen diner with bifolds and a pitched roof can cost between £50,000 and £80,000 or more. In Wickford, we’ve worked on kitchen extensions where the kitchen cost itself made up a big part of the budget. These builds are popular but can be expensive, so planning early helps. Compared to the average cost of extension UK-wide, prices in Essex are fairly competitive when designed with budget in mind.
Rear and wraparound extension cost
A standard 3 metre house extension at the rear is common and often falls under permitted development. Rear extension cost in Chelmsford usually starts around £35,000 to £50,000 for a simple design, but goes higher with open-plan layouts and additions like underfloor heating and LVT flooring. Wraparound extensions cost more overall due to the extra structure, steelwork and foundation work needed around the side, but the cost per m2 is often cheaper than that of a small or medium rear extension. We always check the rules before you spend money on something you might not get planning permission for.
Loft conversion and garage conversion costs
Loft conversions and garage conversions are great ways to make use of space you already have. Costs depend on things like roof height, insulation, and whether plumbing or steel is needed. They’re usually cheaper than building from scratch, but still need proper drawings and checks. We can guide you with free build cost advice based on the m2 rates used by our recommended builders.
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What type of property do you have?
Detached
Semi Detached
Terrace
Bungalow
What affects the cost of building an extension?
House extension costs aren’t just about size. The architectural design, structural engineering layout, and what’s under the ground all make a big difference. In Colchester, we’ve worked on builds where piling added over £10,000 to the job, often because the original house was piled, which means the extension has to be too. That’s not optional.
The spec inside the extension affects the cost too. Kitchen islands can add £6,000 depending on the layout. A downstairs WC adds around £10,000. Smart features like electric opening roof windows can double the price of each unit. Integrated blinds in glazing cost around £2,000 extra. Underfloor heating ranges from £3,000 to £7,000. LVT flooring for the ground floor usually starts at £3,000. Patios and external lighting are also separate from the standard £2k per metre estimate.
We’ve found that in higher-end areas, builders sometimes charge a little more, but the difference isn’t huge. Our recommended builders stay consistent with their prices across Essex, which is one of the many reasons we recommend them to you.
How much do plans and drawings cost?
Planning your build properly starts with the right drawings. The cost of house extension plans varies depending on the size of the project and what services are needed, measured surveys, architectural design, structural calculations, and planning or building control applications. We start at around £1300 and go up from there. Structural calculations are only done after we have received approval from the council.
The bigger the house, the more time we need for the survey and existing plans. That’s why house extension drawings for larger properties usually cost more. Double storey extensions take more work to design than single storey ones, while garage conversions are often the cheapest. Our fees are the same for everyone, so if your project in Tilbury is the same as someone’s in Colchester, Brentwood, or Rayleigh, you’ll be paying the same price. We’ve worked on properties in Southend, Billericay, Chelmsford, Grays and Wickford too. As trusted Essex architects, we keep things fair.
Real examples of house extension costs in Essex
Every build’s different but here are a few house extension cost examples to give you an idea. A 3 metre house extension in Wickford with a simple rear layout, bifolds and LVT flooring came in at around £48,000. Another customer in Billericay added a double storey wraparound with a kitchen, utility and bedroom above. Their cost of house extension work was closer to £115,000.
At Kirkwood we see a big range depending on the finish, access and how much structural work is needed. We give honest advice before you commit to anything.
Chelmsford - New London Road
Single story pitched roof extension
4m x 9m - £64,995+vat
Brentwood – Shenfield Road
Single story pitched roof extension
6.5m x 8.2m - £69,995+vat
Rayleigh – London Hill
Single story pitched roof extension
3m x 6m - £39,995+vat
Basildon – Noak Hill Road
Single story pitched roof extension
7m x 2.8m - £39,800+vat
Southend – Thorpe Bay Gardens
Single story pitched roof extension 35m2 wrap around - £69,995+vat
Billericay – Norsey Road
Single story pitched roof extension 4.5m x 3m - £34,800+vat
Grays – Orsett Road
Single story pitched roof extension
4m x 8m - £63,995+vat
Leigh on Sea – Marine Parade
Single story pitched roof extension 4m x 5.5m - £46,795+vat
Hornchurch – Wingletye Lane
Single story pitched roof extension 4m x 6m - £47,995+vat
Colchester – Lexden Road
Single story pitched roof extension 4.5m x 8m - £68,495+vat
Wickford – London Road
Single story pitched roof extension 4m x 6.8m - £54,395+vat
Canvey Island – Thorney Bay Road
Single story pitched roof extension 4.8m x 7.5m - £68,795+vat
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Use our extension cost calculator to estimate your budget
An extension cost calculator is a good way to get a rough idea of what your build might cost. Most work it out based on the house extension cost per m2, which gives a general range but doesn’t show the full picture. For example, a single storey extension might start around £1,800 per m2, but that won’t include finishes, drainage, or anything outside.
Kirkwood can give you a better estimate than a typical extension cost calculator because we work from real quotes given by the builders we know and trust. We offer free advice based on the type of build you want, the level of spec, and how your site affects the price. Use the calculator as a guide and not a final price. We may have to ask you more details to be able to give an accurate cost.
House extension cost calculator
Tips to keep your house extension cost down
There are loads of ways to keep house extension costs under control. Everyone’s got a budget. Most people can’t afford a gold-plated kitchen, and that’s fine. Overspending on extras is easy, but not always needed. If you’re wondering how much does an extension cost, the answer often comes down to decisions made early on. Things like electric opening roof windows, aluminium bifolds or large sliding doors, underfloor heating, boiler taps, and kitchen islands all push the price up. Roof type, bricks, and window sizes also change the cost. Laminate flooring’s cheaper than LVT. Keeping the kitchen in the same place saves on plumbing and electrics. Designing within permitted development helps too. One of the biggest ways to save is by using our recommended builders. They’re fairly priced across Essex and don’t charge extra just because you live on a nice road.
Design around your budget
We’ll help shape your plans to fit what you can afford, without losing the end result. Small layout changes can save thousands.
Avoid hidden extras
Some builders leave out key costs, then add them on later. Ours don’t. The builders we recommend clearly list what’s included and what’s not, so you know where your money’s going from the start.
Use trusted local builders
Our recommended builders price jobs fairly across Essex. They don’t charge more just because your house is on a nicer road.
Need help planning your extension?
If you’re not sure where to start, we can help. Planning things properly is the best way to keep house extension costs under control and avoid problems later. At Kirkwood, we sort out the drawings, structural details and approvals so your builder knows exactly what they’re working with.
Extension costs in Essex are rising, but making mistakes will always end up costing more. Good design saves money before the build even starts.
Essex | Architectural Design & Planning
Some postcodes we serve in and around Essex
SS11, SS12, CM13,CM14,CM15, CM11, CM12, SS6, SS1, SS2, SS9, SS7, RM16, RM17, RM11, RM12, CM7, CM77, SS8, SS13, SS14, SS15, SS16, CM9, CM1, CM2, CM3, CO1, CO2, CO3, CO4, CM17, CM18, CM19, CM20, CM16, CM4, CM5, CM6, CM8, CM3, CM0, RM18, IG7, IG10, EN9, CO5
Frequently asked questions
How much does a 3 metre house extension cost?
A 3 metre house extension at the rear usually starts around £35,000, depending on the finish and layout. Costs can go higher with extras like bifolds or a new kitchen.
What is the average house extension cost per m2?
Most builds in Essex fall between £1,800 and £2,500 per m2, but that doesn’t include everything. Internal finishes, access, and structural work all add to the price.
How much does a kitchen extension cost?
Kitchen extension cost depends on the layout, fittings and finish. A typical rear kitchen diner often lands between £50,000 and £80,000.
What affects the cost of building an extension the most?
Structure and spec. Things like roof shape, foundation depth, and steelwork can shift the price quickly. Good design keeps the cost of building an extension under control.
What does a small extension cost in Essex?
Small extension cost is usually higher per m2. That’s because you still need drawings, groundwork, structure and sign-off, even if the space is small.
