Double Storey Extensions in Essex - Costs, Planning & Design
A double storey extension is the most space-efficient way to add significant living space to your Essex home. Instead of covering more garden with a single storey build, you extend up — adding two floors from one set of foundations, one set of walls, and one roof. The result is typically 40 to 80 square metres of new space, two extra rooms or more, and a property value increase of 15 to 25 per cent. Kirkwood handles everything: design drawings, planning permission, structural engineering, and the complete build across Essex.
Double Storey Extension Costs in Essex - 2026
Semi-detached rear double storey: from £80,000
Detached rear double storey: from £85,000
Terraced rear double storey: from £75,000
Double storey rear + side combined: from £120,000
All prices include design, structural engineering, planning permission, and build.
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How Much Does a Double Storey Extension Cost in Essex?
The cost of a double storey extension in Essex depends on the configuration, size, and specification. Below are typical price ranges for 2026, covering the most common types across Brentwood, Billericay, Chelmsford, Wickford, Basildon, and surrounding areas. All figures include design, structural engineering, planning permission, and build.
| Extension Type | Cost Essex 2026 | Build Time | Planning | Best For |
| Double storey rear | £80,000 – £140,000 | 16–22 weeks | Usually required | All detached/semi |
| Double storey side | £75,000 – £130,000 | 14–20 weeks | Usually required | Semi-detached/detached |
| Double storey rear + side | £120,000 – £200,000+ | 20–28 weeks | Always required | Detached homes |
| Double storey infill | £70,000 – £120,000 | 14–18 weeks | Usually required | Detached with side gap |
| Ground + first floor rear | £90,000 – £150,000 | 18–24 weeks | Usually required | Most house types |
All figures are guideline costs for a typical Essex home in 2026. Final costs depend on your specific property, ground conditions, and interior specification. Get a free quote for an accurate figure.
WHAT IS INCLUDED IN THE PRICE — AND WHAT IS NOT
Included: Architectural design drawings, structural engineering calculations, planning permission application and management, building regulations package, and the complete build (groundworks, blockwork, steelwork, roof structure and covering, windows, insulation, plasterboard, first fix electrics and plumbing, staircases where applicable).
Not included: Decoration, floor finishes, kitchen or bathroom fitting, fitted wardrobes or furniture. Budget an additional £10,000 to £30,000 depending on the size and specification of the fit-out.
Double Storey Extension Cost by Size
If you know roughly how large you want to extend, here are typical costs for common extension sizes in Essex. These are for a standard double storey rear extension including design and build. The cost per floor is quoted — the total extension adds that floor area on both ground and first floor.
Extension Size (per floor) | Typical Cost (Essex 2026) | What You Get |
3m x 3m per floor (18m2 total) | £65,000 – £95,000 | Compact — ideal for bedroom + bathroom above small kitchen ext. |
4m x 3m per floor (24m2 total) | £80,000 – £115,000 | Most popular size — bedroom + en-suite above open-plan kitchen |
4m x 4m per floor (32m2 total) | £95,000 – £135,000 | Master suite above + large living-kitchen below |
5m x 3m per floor (30m2 total) | £90,000 – £130,000 | Narrow but deep — common on terraced/semi |
5m x 4m per floor (40m2 total) | £110,000 – £155,000 | Large double storey — two bedrooms possible above |
6m x 4m per floor (48m2 total) | £130,000 – £185,000 | Full-width extension — rare but transformative |
* Costs are for Essex in 2026. London-adjacent areas (Hornchurch, Chigwell) sit slightly above these ranges. Prices assume standard ground conditions — allow for additional costs if made-up ground or high water table is discovered during groundworks.
Double Storey Extension Cost by House Type in Essex
The type of property you have affects both the cost and the planning requirements for a double storey extension. Here is a summary of what to expect for each house type in Essex.
House Type | Typical Cost | Party Wall | Notes |
Semi-detached | £80,000 – £140,000 | Party wall required | Most popular in Essex — rear double storey common |
Detached | £85,000 – £160,000 | No party wall | Maximum flexibility — rear, side, or combined possible |
Terraced | £75,000 – £130,000 | Two party walls | Rear-only usually. Both neighbours need party wall notice |
End-of-terrace | £80,000 – £145,000 | One party wall | Can extend to side — similar options to semi |
Semi-Detached Houses
Semi-detached homes are the most common candidate for double storey rear extensions in Essex. The extension adds a new floor above the rear of the property typically a bedroom and bathroom above an enlarged kitchen or living space. Most semi-detached double storey rear extensions require full planning permission from the local council, as they exceed permitted development limits. Party Wall Agreements with the adjoining neighbour are required before work can begin. In Billericay, Chelmsford, Brentwood, and Wickford, double storey rear extensions on semis are extremely common and councils are generally supportive of well-designed applications.
Detached Houses
Detached homes offer the most flexibility for double storey extensions. Without shared walls to consider, extensions to the rear, side, or a combination of both are all viable. There are no party wall obligations. A full double storey side extension , creating a new wing alongside the existing house, is particularly popular on detached properties in Brentwood, Hutton, and Rayleigh. Cost typically ranges from £85,000 to £160,000 depending on size and specification.
Terraced Houses
Terraced houses can have double storey rear extensions, though the options are more limited than for semi-detached or detached properties. The extension is restricted to the rear — side extensions are not possible in most cases. Two Party Wall Agreements will be needed with both adjoining neighbours before building work starts. Cost on a terraced house typically ranges from £75,000 to £130,000. In Southend, Leigh-on-Sea, and parts of Chelmsford, double storey rear extensions on terraced homes are common and well-accepted by planners.
Single Storey vs Double Storey Extension – Which Is Better Value?
Whether a single or double storey extension is the right choice depends on your budget, how much space you need, and your long-term plans. Here is a clear comparison.
| Single storey extension | Double storey extension | |
| Cost | £35,000 – £85,000 | £80,000 – £200,000+ |
| Space added | One new room or enlarged existing space | Two floors of new space — typically 40–80m2 |
| Build time | 4–6 months total | 8–14 months total |
| Planning permission | Often not required (permitted development) | Almost always required |
| Value added | 5–15% uplift typically | 15–25%+ uplift — best return per pound spent |
| Garden lost | Yes — some garden lost | Yes — same footprint as single storey |
| Best for | Adding one room, tight budget, quick project | Adding 2+ rooms, maximising value, long-term |
The key point is this: a double storey extension costs roughly 60 to 80 per cent more than a single storey of the same footprint, but delivers twice the floor space. On a cost-per-square-metre basis, a double storey extension almost always delivers better value than building single storey. It is also more cost-effective than two separate projects — building the ground floor now and adding above later is significantly more expensive than building both floors at once.
For Essex homeowners where the difference in value between a three-bedroom and a four-bedroom home is significant — Billericay, Brentwood, Chelmsford, Leigh-on-Sea — a double storey extension frequently adds more in property value than it costs to build.
Do You Need Planning Permission for a Double Storey Extension?
Almost all double storey extensions require full planning permission. Unlike single storey rear extensions — which often qualify as permitted development — double storey extensions almost always exceed permitted development limits due to their size and impact on neighbouring properties.
WHEN PLANNING PERMISSION IS REQUIRED – Any double storey rear extension (exceeds PD height limits in most cases) – Any double storey side extension – Combined rear and side double storey extensions – Extensions that exceed the 3-metre rear boundary rule for two-storey builds – Extensions on properties in conservation areas – Any property that is listed | THE LIMITED EXCEPTIONS — RARELY APPLICABLE – A double storey rear extension may theoretically qualify as PD if it is at least 7 metres from the rear boundary — rare on typical Essex plots – It must not exceed the height of the existing house – It must not have side-facing windows at first floor level – Materials must match the existing property exactly – The property must not be in a conservation area or Article 4 area |
Planning Permission Timeline for Double Storey Extensions in Essex
A standard householder planning application in Essex takes 8 to 10 weeks to be determined. Kirkwood prepares all drawings, plans, and the application itself — the client does not need to interact with the council directly. We manage the process and keep you updated. If pre-application advice is sought from the council first (recommended for larger or more complex schemes), allow 4 to 6 additional weeks.
Essex councils Kirkwood regularly works with for double storey extension applications include: Chelmsford City Council, Brentwood Borough Council, Basildon Council, Rochford District Council, Castle Point Borough Council, Southend-on-Sea City Council, and the London Borough of Havering (Hornchurch, Upminster, and surrounding areas).
LAWFUL DEVELOPMENT CERTIFICATE — IS IT NEEDED FOR A DOUBLE STOREY EXTENSION?
Because virtually all double storey extensions require full planning permission, a Lawful Development Certificate is not relevant in most cases. A formal planning approval is what you need, and that is what Kirkwood applies for on your behalf.
If your project is one of the rare cases that might qualify as permitted development, Kirkwood will assess this and apply for a Lawful Development Certificate where appropriate but this will be discussed during the free consultation.
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Building Regulations for Double Storey Extensions
All double storey extensions must comply with building regulations, regardless of whether planning permission was required. Building regulations set the standards for structural safety, fire protection, insulation, ventilation, staircases, and drainage. Kirkwood prepares the full building regulations package — this is the detailed set of drawings and calculations your builder works from during construction.
Structural Engineering
A double storey extension requires detailed structural calculations. Steel beams are almost always required where existing walls are removed to open up the ground floor. The new foundations must be designed to carry the load of two floors. Floor joists and wall structures must be specified correctly. Kirkwood includes structural engineering as part of its complete design service — the architectural and structural packages are produced together, which speeds up the process and avoids coordination issues.
Party Wall Agreements
If you share a wall with a neighbour — which applies to all terraced and semi-detached homes — you must serve a Party Wall Notice on your neighbour before building work starts. If your neighbour agrees in writing, work can proceed. If they appoint their own surveyor, you will be liable for those surveyor fees. Kirkwood advises on the party wall process and can recommend party wall surveyors where needed. Party Wall Agreements are separate from planning permission and must be in place before your builder starts on site.
Fire Safety
Building regulations for two-storey extensions include specific fire safety requirements. Fire-resistant doors, interlinked smoke alarms, and appropriate wall construction to contain fire spread are all required. The staircase that connects the new upper floor to the rest of the house must form a protected escape route. Kirkwood details all of this in the building regulations package so your builder knows exactly what is required.
Insulation and Energy Performance
New extensions must meet current energy efficiency standards. Walls, roof, and floor must all achieve required U-values. In 2026 this means a wall U-value of 0.18 W/m2K or better, and a roof U-value of 0.15 W/m2K or better. A well-insulated extension typically costs less to heat than the rest of the house and adds to the overall EPC rating of the property.
How the Process Works - From First Call to Finished Build
Kirkwood manages the entire process from the first conversation through to the completed extension. Here is what happens at each stage.
1 | Free consultation We visit your property, assess the space, discuss what you want to achieve, and give you an honest cost estimate. We research planning records and neighbouring developments before the visit so we arrive with answers, not just questions. No charge, no obligation. |
2 | Design and drawings We carry out a full measured survey and produce architectural drawings — floor plans, elevations, and sections. You review and approve. Most projects are finalised in the first or second draft. Typically takes 4–6 weeks. |
3 | Planning application We prepare the planning application, submit it to your local council, and manage it through to decision. Standard determination time is 8–10 weeks. We respond to any queries from the council and negotiate conditions where needed. |
4 | Building regulations We prepare the full building regulations package including structural engineering calculations. This is submitted concurrently with planning, so there is no additional wait. Building regulation approval typically takes 5–8 weeks. |
5 | Party wall and pre-start While planning and building regs are processed, any required Party Wall Notices are served. We also connect you with trusted Essex builders from our network who price our designs — you choose your builder, we support the process. |
6 | Build Groundworks, structure, roofing, first fix, plastering, and finishing. A standard double storey rear extension takes 16 to 22 weeks to build. Kirkwood is available throughout to answer technical queries from your builder. |
7 | Completion and sign-off Building inspector sign-off and final completion certificate. This is a legal document and is important for when you sell. Your extension is complete and ready to use. |
Total project timescale from first call to completion: approximately 10 to 14 months for a standard double storey extension in Essex. Projects that need planning permission (which is almost all of them) take longer than single storey permitted development projects.
What Can You Do with a Double Storey Extension?
A double storey extension adds two floors of usable space from the same footprint. Here are the most popular uses across Essex homes.
Ground Floor Options
- Open-plan kitchen-diner — the most popular choice. Remove the back wall and extend to create a large, light-filled kitchen with dining and living area, connected to the garden.
- Additional living room — create a separate family room, snug, or playroom at the rear while the original living room remains.
- Utility room and extended kitchen — add a proper laundry and storage space that takes the pressure off the main kitchen.
- Downstairs WC and cloakroom — often incorporated into a double storey side extension alongside other uses.
First Floor Options
- Master bedroom with en-suite — the most common use of the first floor addition. Creates a proper master suite that the original house did not have.
- Additional bedroom — converts a three-bedroom house into a four-bedroom home. In most parts of Essex, this adds significantly more value than the cost of the extension.
- Home office — a dedicated work-from-home space at the rear of the house with natural light and separation from the main living areas.
Children’s bedroom and bathroom — gives growing children their own space and frees up the existing bedrooms.
Double Storey Extensions Across Essex - Areas We Cover
Kirkwood provides double storey extension design and build services across Essex and parts of East London. We have completed projects across Brentwood, Billericay, Chelmsford, Wickford, Basildon, Rayleigh, Leigh-on-Sea, Southend-on-Sea, Hornchurch, Benfleet, Braintree, Chigwell, Colchester, Grays, and surrounding areas.
If you are not sure whether we cover your area, call 01268 944233 or submit a quick quote online. We confirm within 24 hours.
Double Storey Extension - Frequently Asked Questions
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How much does a double storey extension cost in Essex?
A double storey extension in Essex costs between £80,000 and £200,000 in 2026, depending on size, house type, and specification. A standard double storey rear extension on a semi-detached home typically costs £80,000 to £140,000 including design, structural engineering, planning permission, and build. Get a free quote for an accurate figure for your specific property.
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Do I need planning permission for a double storey extension?
Almost always yes. Double storey extensions almost always require full planning permission because they exceed permitted development size and height limits. The rare exception is an extension at least 7 metres from the rear boundary, not exceeding the existing roof height, with no side-facing upper-floor windows — but this is uncommon on typical Essex plots. Kirkwood assesses your specific property and manages the planning application as part of its service.
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How long does a double storey extension take?
From first consultation to completion, a double storey extension in Essex takes approximately 10 to 14 months. This covers 4–6 weeks of design, 8–10 weeks for planning permission, 5–8 weeks for building regulations (running concurrently), and 16–22 weeks of building work. Kirkwood provides a full week-by-week programme before work begins.
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What is the difference between a single storey and double storey extension?
A single storey extension adds one level of floor space and typically qualifies as permitted development. A double storey extension adds two floors from the same footprint — double the space, slightly higher cost, but far better value per square metre. A double storey almost always requires planning permission. The value uplift from a double storey is typically 15–25%, compared to 5–15% for a single storey.
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Is a double storey extension worth it?
For most Essex homeowners, yes. A double storey extension converts a 3-bedroom home into a 4-bedroom home, which commands significantly more on the market in areas like Billericay, Brentwood, and Chelmsford. The cost-per-square-metre of a double storey is lower than a single storey because the foundations, walls, and roof are shared across two floors. The return on investment is typically strong.
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Can I build a double storey extension on a semi-detached house?
Yes. Semi-detached homes in Essex are the most common property type for double storey rear extensions. You will need a Party Wall Agreement with your adjoining neighbour and planning permission from your local council. Both are standard processes that Kirkwood manages as part of the project. Costs typically range from £80,000 to £140,000 for a rear double storey on a semi.
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Do I need a structural engineer for a double storey extension?
Yes. All double storey extensions require structural engineering calculations. Steel beams are almost always needed where existing walls are removed, and the foundations must be designed to support two floors of new structure. Kirkwood includes structural engineering as part of its complete design service — architectural drawings and structural calculations are produced by the same team.
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How much does a double storey extension add to property value?
A well-designed double storey extension typically adds 15 to 25 per cent to property value in Essex. Converting a 3-bedroom semi into a 4-bedroom home can add £50,000 to £120,000 in value depending on location. In high-demand areas like Brentwood and Billericay, the value added often exceeds the cost of the extension.
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Can I build a double storey side extension?
Yes. A double storey side extension uses the space alongside your home to add a new wing — typically with a bedroom or study above and a utility room, home office, or garage below. Side extensions almost always require planning permission. They are most practical on semi-detached and detached homes where there is an unused strip of land to the side.
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What is a double storey rear and side extension?
A double storey rear and side extension sometimes called a double storey wraparound combines a rear extension and a side extension into a single two-storey structure. It is the most ambitious single project type for a domestic property and can add 60 to 100 square metres of space across two floors. It always requires planning permission and typically costs £120,000 to £200,000 or more in Essex in 2026.
