How Much Does a House Extension Cost in Cambridge and Cambridgeshire?

Updated August 2026. Published UK guidance suggests using roughly £1,800–£3,000 per m² as an early budgeting benchmark for a single-storey house extension. For a 20m² extension, that is simple arithmetic of about £36,000–£60,000 before VAT. It is not a Cambridge quotation: access, structural design, ground conditions, drainage, glazing, finishes and work to the existing house can materially change the price.

This guide explains how to build a realistic early budget for a house extension in Cambridge and Cambridgeshire, what published figures may leave out, and what information a builder needs before providing a detailed quotation.

All figures below are budgeting illustrations, not offers or quotations. A written, site-specific quotation based on current drawings and a defined specification supersedes any general £/m² guide.

Extension costs at a glance

The Federation of Master Builders’ 2026 guide gives a broad UK single-storey extension benchmark of £1,800–£3,000 per m². Checkatrade’s June 2026 guide publishes the same band and states that its figure excludes VAT.

The table below multiplies that £1,800–£3,000 benchmark by the new floor area and then shows the effect of adding standard-rate VAT at 20%. It does not turn a generic rate into a local quote.

Do not use the final column as an all-in project budget. Depending on the source and the project, £/m² figures may exclude design and engineering, planning and Building Control charges, a kitchen or bathroom, premium glazing, alterations inside the existing house, drainage or utility work, landscaping, temporary accommodation and contingency.

Smaller extensions are not always cheaper per square metre. Site setup, skips, welfare, structural design, a new opening, drainage and the basic foundation and roof work still have to be provided, but those fixed costs are spread across fewer square metres.

A simple way to form an early extension budget

Rather than relying on one online “extension calculator” result, build the budget in separate lines:

Our own house extension cost calculator does exactly that. It applies the benchmark below, adds VAT, the planning fee and a contingency, and leaves the four figures nobody can estimate from a web page as blanks for you to fill in once you have them.

Indicative construction allowance
+ design, surveys and engineering
+ planning, Building Control and other approvals
+ kitchen, glazing and other client-selected items not in the construction allowance
+ VAT where applicable
+ contingency
= early project budget

This makes assumptions visible. If a later design change adds a large structural opening, bespoke glazing or a more complex roof, you can see which part of the budget needs to change.

Worked example: a 20m² single-storey extension

Using the published £1,800–£3,000 per m² band gives £36,000–£60,000 before VAT. Applying 20% VAT produces £43,200–£72,000. Design, structural engineering, application and Building Control charges, a kitchen, unusually large glazing, landscaping and contingency may still sit outside that illustration.

The useful conclusion is not that a 20m² Cambridge extension “costs” one of those totals. It is that the homeowner should test the project against a broad construction band, list the omitted items, and then replace assumptions with scoped quotations.

Worked example: a 30m² kitchen-dining extension

The same arithmetic gives £54,000–£90,000 before VAT or £64,800–£108,000 with 20% VAT. The spread is wide because “kitchen extension” can describe very different projects. A straightforward room with standard openings is not comparable with extensive internal remodelling, a bespoke kitchen, underfloor heating, large sliding doors, rooflights and premium finishes.

For a meaningful budget, keep the building work, kitchen supply, appliances and specialist finishes as separate lines until the quotation confirms exactly what is included.

What does an extension price per square metre include?

There is no universal definition. One publisher may describe a weather-tight shell; another may assume a more complete room. Before comparing any figures, check the stated scope for:

• site setup, access protection, scaffolding, skips and waste removal;
• demolition, excavation, foundations and ground-floor construction;
• drainage alterations and utility work;
• brickwork, blockwork, insulation and matching materials;
• structural steel, temporary support and the opening into the existing house;
• roof construction, roof coverings, rooflights and rainwater goods;
• windows, external doors and large glazed openings;
• electrical, plumbing and heating allowances;
• plastering, joinery, decoration and floor finishes;
• kitchen, sanitaryware, tiling, appliances and specialist fittings;
• external making good, patios, paths and landscaping;
• professional and statutory fees; and
• VAT.

A low headline rate may be a shell-only figure with many of these items excluded. A higher quotation may cover a substantially more complete and clearly specified scope. Our guide to what a builder’s quotation should include explains how to compare proposals on the same drawings, revisions and specification.

How the type of extension changes the budget

Single-storey rear extensions

A rear extension may look simple on a plan, but the cost depends on the opening formed in the existing wall, foundation and drainage design, roof form, glazing, access and the amount of internal alteration. Creating one large kitchen-dining space can require substantial temporary support, steelwork and service changes.

Side-return and wraparound extensions

Narrow access, boundaries, drainage and the junction between new and existing roofs can make side-return work more complex than its floor area suggests. A wraparound layout adds more external wall, roof and glazing and may involve a larger area of remodelling inside the original property.

Two-storey extensions

A two-storey extension does not automatically cost twice as much as a single-storey extension of the same footprint. Some foundation, site-setup and roof costs serve both floors, so the rate per square metre can be lower. The total project is nevertheless larger, and structural requirements, scaffolding, first-floor alterations, services and planning considerations can add complexity. It should be priced from the total new floor area and developed design, not from a blanket percentage.

Kitchen and bathroom extensions

Rooms with significant plumbing, electrical work, ventilation, cabinetry, appliances, tiling and specialist finishes usually need larger product allowances than an unfitted living space. Ask whether the builder’s price includes supply, installation, connections and decoration—or only the building shell around customer-supplied items.

Why Cambridge and Cambridgeshire extension costs vary

Access, parking and material storage

A terraced or tightly enclosed Cambridge property can require materials and spoil to travel through a narrow route. Parking restrictions, limited skip positions and little on-site storage may affect labour, deliveries and waste handling. A more open Cambridgeshire site may be easier, but every property needs an access check.

The existing house and structural opening

Property age, previous alterations and the construction of the existing walls and foundations all matter. A wide opening between the house and extension can require engineered beams or frames, temporary works and local strengthening. The making-good required in occupied rooms should also be defined.

Ground, trees, drainage and utilities

Foundation design is site-specific. Trees, drains, soil investigations, previous structures and nearby foundations may affect the engineer’s solution. Drain diversions, build-over requirements and incoming-service changes can also move a project away from a generic square-metre allowance.

Planning and design constraints

Conservation areas, listed buildings, planning conditions and the need to match existing Cambridge brickwork or other finishes can influence the design and procurement. Some extensions may be permitted development, but that is a legal planning test rather than a cost assumption. Read our Permitted Development guide and Planning Permission guide for the main routes.

Specification and selections

Aluminium glazing, bespoke doors, rooflights, specialist brick or stone, underfloor heating, fitted furniture and the kitchen specification can create a large difference between two extensions of identical size. Resolve important selections early and record allowances for anything not yet chosen.

Costs to allow for outside the builder’s price

Every quotation is different, so exclusions must be checked in writing. Common separate costs include the following.

Architectural design and surveys

An architect or architectural designer may charge a fixed sum, a percentage or time-based fees depending on the scope, complexity and service. There is no single official domestic fee scale. Ask for a written proposal stating which surveys, design stages, applications, technical drawings, tender information and site-stage services are included.

Structural engineering

An engineer may need to design beams, columns, foundations and structural details and provide calculations for Building Control. Fees vary with the design and investigations required. The Institution of Structural Engineers’ homeowner guidance explains what a structural engineer may provide and what to check when appointing one.

Planning and lawful-development fees

From 1 April 2026, the statutory fee in England for a full householder application to enlarge, improve or alter one dwelling is £548 when that application is required. A proposed Lawful Development Certificate is normally half the relevant full fee—£274 in this example—and the prior-approval fee for a larger home extension is £249. Check the current route and fee with the local planning authority or the Planning Portal before applying.

The Planning Portal also applies an online service charge to qualifying fee-bearing applications. In Greater Cambridge, optional householder pre-application advice is a separate service; the current council page lists the available levels and fees.

Building Regulations approval

Planning and Building Regulations are separate. An extension will commonly require Building Regulations approval even when planning permission is not required. There is no single national Building Control fee: charges depend on the work, floor area and inspection route. GOV.UK explains the approval routes; obtain the current project-specific charge from the relevant local authority service or registered building-control approver.

Party Wall matters

The Party Wall etc. Act may apply to work on or near a boundary, work to a shared wall or structure, or certain excavations near a neighbour’s foundations. It is separate from planning and Building Regulations. Whether surveyor fees arise depends on the work and the adjoining owner’s response, so a universal “Party Wall cost” is misleading. See the official Party Wall guidance and take advice early enough to serve any required notice.

VAT

Most ordinary work to an existing home, including an extension, is standard-rated at 20% VAT when supplied by a VAT-registered contractor. Check every quotation and professional-fee proposal states whether VAT is included. A relief applying to a qualifying item does not automatically reduce the VAT rate for the whole extension. See HMRC’s VAT guidance for building work.

Contingency and living arrangements

The FMB recommends ring-fencing at least 10% of the overall project cost for genuine unforeseen issues and considering 15% or more for older homes or structurally involved projects. Contingency is not a hidden allowance for upgrades. Temporary kitchen arrangements, storage, protection of possessions and accommodation should be considered separately where relevant.

Will an extension add value?

An extension can improve space and layout, but no builder can guarantee that the property’s market value will rise by more than the project cost. The result depends on local comparable sales, the quality and usefulness of the new space, bedroom and bathroom balance, plot and garden, energy performance and the ceiling price for the road.

The Office for National Statistics extension value calculator is a useful external starting point. It is an estimate based on data, not a valuation or a guarantee. For a purchase or investment decision, seek advice from a local RICS-registered valuer or other suitably qualified property professional.

How to get a useful extension quotation

A builder can discuss feasibility from early information, but a dependable quotation needs a defined scope. For an initial project review, provide:

• the property postcode and brief description of the existing house;
• the latest drawings, with drawing numbers and revision dates;
• the planning or lawful-development position;
• Building Regulations information and structural details available so far;
• a specification or clear notes on the intended finish;
• known access, parking, drainage or boundary constraints;
• the preferred start period;
• an honest working budget; and
• which kitchens, glazing, finishes or other products you expect to supply.

Give every builder the same current information. Then compare inclusions, exclusions, allowances, VAT, programme, payment stages and the process for variations—not just the total at the bottom.

Relevant extension projects

Real projects are more useful than generic cost promises because they show how access, structure, glazing and refurbishment interact. Browse our Cambridge and Cambridgeshire building projects and our house extension service to see the type of information and completed-work evidence available.

Project examples include the Highfields Caldecote glazed rear extension, where glazing and the connection to the existing house are important cost considerations, and the Clay Street extension and refurbishment, which shows why the extension and work within the original property must be scoped together. For two-storey scope, the Almoners Avenue two-storey extension in Cambridge documents the construction sequence—foundations, structural steel, cavity brickwork and roof—stage by stage.

House extension cost questions

How much does an extension cost per square metre in Cambridge?

There is no authoritative public Cambridge rate that can replace a quotation. Current UK guides suggest roughly £1,800–£3,000 per m² for early single-storey-extension budgeting. Treat that as a broad feasibility benchmark; a Cambridge site’s access, design, structure and specification determine the actual price.

How much might a 20m² extension cost?

Multiplying 20m² by £1,800–£3,000 gives £36,000–£60,000 before VAT. At 20% VAT, the arithmetic becomes £43,200–£72,000. Professional and statutory fees, client-selected products, other exclusions and contingency may still need to be added.

How much might a 30m² extension cost?

The same benchmark produces £54,000–£90,000 before VAT, or £64,800–£108,000 after adding 20% VAT. This is an illustration, not a statement that all 30m² Cambridge extensions fall within that range.

Does a two-storey extension cost twice as much?

Not necessarily. Both floors share some foundation, site and roof costs, which can reduce the rate per square metre. The total new floor area is larger, however, and the structural design, scaffolding, services and alterations may be more involved. Use developed drawings for a meaningful comparison.

Are kitchens and bathrooms normally included?

There is no standard rule. A price may exclude the products, include only installation, or include defined supply allowances. The quotation should separate cabinetry, appliances, sanitaryware, tiling, plumbing, electrics, ventilation and decoration clearly enough to understand the scope.

Can a builder quote before drawings are complete?

A builder may provide early feasibility guidance, but unresolved drawings, structure and selections create assumptions. A detailed quotation is more reliable when the design, revisions, specification and known site conditions are sufficiently developed.

Do I need planning permission?

Some domestic extensions can be permitted development if every relevant condition and limitation is met; others need planning permission. Conservation areas, listed status and previous changes can affect the route. Confirm the position for the property, and remember that Building Regulations approval is separate.

How much contingency should I allow?

Current FMB guidance recommends at least 10% of the total project cost, with 15% or more considered for older homes or structurally involved work. The right allowance depends on how much investigation and design has been completed and the remaining uncertainties.

How long does an extension take?

There is no responsible universal duration. Design, approvals and procurement happen before construction; the build programme then depends on size, complexity, structural work, access, product lead times and changes. Ask for a project-specific programme and identify the decisions or approvals on which it depends.

If you are weighing an extension against other ways of adding space, our loft conversion cost guide sets out the 2026 benchmarks by conversion type, and our new build house cost guide covers building from scratch — including the VAT position, which is very different from an extension.

How this guide was prepared

We reviewed current August 2026 public guidance from the Federation of Master Builders, HMRC, GOV.UK, the Planning Portal, Greater Cambridge Shared Planning, the Institution of Structural Engineers and the Office for National Statistics. The worked figures are transparent multiplication of a published UK benchmark; they are not described as Prestige’s average price or as an average Cambridge price.

Get a clear next step for your extension

Tell us what you want the new space to do and share any plans you already have. We’ll explain the most useful next step and arrange a site visit where appropriate.

What we need to quote

• Property address and brief
• Current drawings
• Planning and structural information
• Preferred start window and budget range

Request a quote or view completed extension projects.

More on planning, costs and contracts in our building guides. Some homeowners spread the cost — our building project finance page explains the options and who provides them.

Highfields Caldecote, South Cambridgeshire. Completed 2025.

A single-storey glazed rear extension with cavity brickwork, structural steel, a warm flat roof and a roof lantern.

Prestige built it from the foundations to decoration, working from JPT Design’s drawings and Gawn Associates’ structural calculations.

See the Highfields Caldecote extension

Last reviewed: 31 August 2026. Prices, fees, tax rules and approval routes can change. Check current official information and obtain project-specific professional advice and written quotations before committing to work.