An extension budget can go wrong before the drawings are finished if one national rate is treated as a quotation. Published 2026 guidance puts a standard single-storey extension at about £1,800 to £3,000 per square metre, but that range says little about the condition of a particular Rugby home. Foundations, drainage, access, steelwork, glazing and changes to the existing rooms can matter as much as the new floor area. The useful question is not simply, ‘What does an extension cost?’ It is, ‘What finished result does this figure actually buy?’ This guide separates benchmark figures from project costs, explains planning and building control checks, and shows how to compare quotations on the same basis. It also flags figures that exclude VAT or rely on older worked examples. Use the ranges to test whether an early budget is plausible, then replace them with a property-specific design, scope and quotation before making a commitment.
The short answer
Start with the scope, not one headline figure
Current UK guides place a standard single-storey extension at roughly £1,800 to £3,000 per square metre. Checkatrade's 20 square metre example is £40,000 to £56,000 before VAT. Those figures are planning benchmarks, not a verified Rugby rate or a Belmax quotation. Ground conditions, access, structural openings, glazing, services, professional fees and the finish determine the actual total.
What current 2026 extension guides say
Current national guides suggest £1,800 to £3,000 per square metre, but they do not price a specific Rugby property.
Published guides are most useful as an early sense check. They cannot price a particular home because they do not know the ground, drainage, structure, access or specification.
The Federation of Master Builders gives a broad UK range of £1,800 to £3,000 per square metre for a single-storey extension. Checkatrade uses the same broad range for a full extension and publishes the examples below. Its size examples are labelled as last updated in May 2024 and exclude VAT, so they are a sense check rather than a current Rugby price list.
| Illustrative floor area | Checkatrade range, excluding VAT | Average in the guide | What the figure cannot tell you |
|---|---|---|---|
| 20m² | £40,000 to £56,000 | £48,000 | Whether the kitchen, large glazing, professional fees and work to existing rooms are included. |
| 30m² | £60,000 to £84,000 | £72,000 | The effect of ground conditions, drainage, access, structure and specification. |
| 50m² | £100,000 to £140,000 | £120,000 | Whether the project is a simple shell or a complete finished living space. |
How to use a published range
Use the lower and upper figures to test an early idea, not to set the contract budget. First multiply the proposed floor area by the guide range. Then list the major features that are easy to miss, such as a large structural opening, rooflights, kitchen work, drainage changes and making good inside the existing house.
If a quotation seems unusually low or high, look to the written scope for the explanation. The figures may cover different specifications, access conditions or alterations to the existing home.
Check the guidance: Federation of Master Builders cost guide, Checkatrade extension cost guide.
Checkatrade labels these size examples as last updated in May 2024. They are national planning benchmarks, not Rugby-specific prices and not Belmax prices.
Why cost per square metre is only a starting point
The connection to the existing house can influence the budget as much as the new floor area.
Cost per square metre is convenient, but an extension is not priced like an empty box. Some of the largest costs are created by how the new space joins the existing house.
A smaller project can have a higher cost per square metre because site setup, foundations, drainage, a roof and a structural opening are still required. A larger, simple rectangle may spread those fixed costs across more floor area.
- Ground conditions and the foundation design needed for the site.
- Drainage routes, manholes and services beneath the proposed footprint.
- Access for labour, materials, skips and machinery.
- The size of the opening into the existing home and the structural support it needs.
- Roof shape, rooflights, lanterns, windows and glazed doors.
- Heating, electrics, plumbing, ventilation and any boiler changes.
- Kitchen, utility or bathroom fittings and the chosen level of finish.
- Making good and reconfiguring rooms beyond the new extension itself.
The existing house changes the job
Opening a rear wall to create one large kitchen and living space can require substantial temporary support and permanent steelwork. Moving a boiler, altering drainage or changing levels between old and new floors can also add work that is not visible on a simple area calculation.
A Rugby terrace with no side access may need materials and waste moved through the house. That constraint is not captured by a national square-metre rate.
Bottom line: Use floor area to frame the conversation, then price the design, existing structure and site conditions together.
Check whether the figure is for a shell or a finished room
A quotation is only comparable when it defines the same finished result and the same responsibilities.
Two figures can look far apart while describing different jobs. A shell may cover the foundations, external walls and roof but leave glazing, services, plastering, flooring, decoration and the kitchen outside the price. A full build may include some or all of those items.
Ask for a written scope that identifies inclusions, exclusions and any provisional sums. If the extension will become a kitchen or bathroom, confirm who supplies and fits every major item. A clear division of responsibility matters more than the label attached to the package.
What a shell figure may leave out
The term shell has no universal definition. It might stop at a weather-tight structure, or it might include first-fix services and plastering. Ask whether external doors and windows, internal openings, insulation, screed, drainage connections and making good are included.
What makes a finished-room figure useful
A finished-room quotation should connect the drawings to a specification, with clear allowances for glazing, rooflights, flooring and joinery. If a kitchen is supplied separately, define who measures, coordinates it and connects the services.
Costs to plan beyond the main building quotation
Professional fees, VAT, fittings and contingency can sit outside the headline construction figure.
The build total is only one part of the project budget. Depending on the property and design, separate costs may include measured surveys, design drawings, structural calculations, planning work, building control, party wall advice and specialist surveys.
Which? advises allowing a contingency of around 10 to 15 percent for unexpected additional costs. That is a planning buffer, not an invitation to accept an unclear quotation. The scope, assumptions and route for agreeing changes should still be written down before work starts.
- Confirm whether quoted figures include VAT where applicable.
- Separate professional and approval fees from the construction price.
- Budget for the kitchen, appliances, flooring, decoration and external works if they are excluded.
- Allow for temporary storage or accommodation if the affected rooms cannot stay in use.
- Keep a contingency that is controlled by you, rather than treating it as part of the builder's price.
Keep the contingency separate
Keep the contingency under your control. Record agreed changes and maintain a running forecast so unexpected work does not quietly consume the budget.
Check the guidance: Which? 2026 extension cost guidance.
Planning an extension in Rugby
Permitted development can apply, but the house, previous additions and every design limit must be checked.
Some house extensions can be built under permitted development rights when every relevant limit and condition is met. Planning Portal guidance says the rules depend on matters including the original house, position, height, boundaries, previous additions and whether the property is on designated land. Larger single-storey rear extensions can also require prior approval even when full planning permission is not required.
Rugby Borough Council warns that permitted development rights may have been removed from a property. It offers formal pre-application advice for householder extensions or outbuildings for £115 from 1 April 2026, and points homeowners towards a Certificate of Lawfulness when a formal decision on permitted development is needed.
Do not assume that a neighbour's extension establishes what your property can build. Previous additions, planning conditions, listed status, conservation controls and the specific design can change the route.
Common limits for a single-storey rear extension
Planning Portal currently states that a single-storey rear extension must not extend beyond the original rear wall by more than four metres for a detached house or three metres for another house under the standard limits. The larger-home-extension process can increase those depths to eight metres and six metres respectively, subject to prior approval and the other conditions.
The extension must also stay within the rules on maximum height, eaves near a boundary and the amount of land covered around the original house. Side extensions have separate width and height limits. These are national rules for houses in England, not confirmation that a particular Rugby proposal qualifies.
Check the guidance: Planning Portal extension rules, Rugby Borough Council pre-application advice.
Bottom line: Confirm the planning route against the property before treating the floor plan or programme as fixed.
Building regulations and party wall checks are separate
Planning permission, building regulations and party wall duties are separate checks with different purposes.
Building regulations approval is different from planning permission. GOV.UK states that the regulations cover the construction and extension of buildings. The technical work can include structure, fire safety, insulation, ventilation, drainage, electrics and glazing, depending on the proposal.
A party wall process is separate again. Government guidance says notice may be required when work involves a shared boundary or party wall, or when excavation will go below and near a neighbour's foundations. Whether it applies depends on the proposed work and the neighbouring property, so obtain project-specific advice early enough for the required process.
Put responsibilities into the project plan
Confirm who will prepare structural calculations, submit the building regulations application, answer technical queries and arrange inspections. The quotation should also say whether application and inspection charges are included. Completion paperwork matters, so agree who will collect and hand over certificates for the work and installed systems.
If party wall advice is needed, allow for the statutory process before setting a start date. Planning approval or a lawful-development decision does not replace the notices required by the Party Wall etc. Act 1996.
Check the guidance: GOV.UK building regulations approval, GOV.UK party wall guidance.
How to compare extension quotations properly
Compare at least three detailed quotations against one set of drawings, specifications and assumptions.
Give each builder the same drawings, specification and list of assumptions. Then compare the proposed scope line by line. Which? recommends obtaining at least three detailed quotations and checking that VAT is included.
The lowest total is not automatically the lowest final cost. A cheaper quotation may simply leave more decisions, materials or professional responsibilities outside the price.
- Check the drawings and specification used to prepare each quotation.
- Confirm foundations, drainage, steelwork, scaffolding and waste removal.
- Identify who supplies glazing, sanitaryware, kitchens, flooring and finishes.
- List professional fees, applications and inspections as included or excluded.
- Ask how provisional sums and changes will be priced and approved.
- Agree payment stages against visible progress or defined milestones.
- Confirm the proposed programme and what must happen before the start date.
Normalise the figures before choosing
Create a simple comparison sheet with one row for each part of the scope. Move excluded items into a separate client budget, mark uncertain allowances and add VAT where applicable. This exposes whether the apparent saving is real or has simply moved outside the quotation.
Also compare how each builder handles changes, payment stages, site protection and communication. Those details affect budget control even when they do not change the initial total.
Check the guidance: Which? advice on extension quotations.
Bottom line: The strongest quotation is the clearest account of the agreed result, responsibilities and route for managing change.
What Belmax needs before giving a useful quotation
A useful first brief combines the property, intended room, available documents and the finish you expect.
A postcode, a few photographs and a clear description of how the new room should work are enough for the first conversation. If you already have drawings, structural information or a planning decision, include them.
A meaningful quotation needs an agreed design and specification. It should explain the building work, the connection to the existing home, the main finishes and the items that remain your responsibility. That is the point where a broad market range becomes a project-specific figure.
Prepare these details for the first discussion
Explain what is not working in the current home, who will use the new space and any budget or timing constraint that shapes the design. Photographs of the rear elevation, side access, garden, nearby drains and the rooms that will connect to the extension make the first review more useful.
If several options are possible, rank the essentials separately from preferences so the design has a clear priority.
Bottom line: A clear brief makes it easier to test feasibility and reduces the number of assumptions hidden inside an early figure.
Frequently asked questions
How much does a house extension cost in Rugby?
There is no single verified Rugby rate. Current UK guides place a standard single-storey extension at roughly £1,800 to £3,000 per square metre. Checkatrade's 20 square metre example is £40,000 to £56,000 before VAT. Use those figures for early planning only. Ground conditions, drainage, access, structural openings, glazing, changes to existing rooms, professional fees and the chosen finish determine the project-specific quotation.
How much should I budget for a 20m² extension?
Checkatrade shows £40,000 to £56,000 before VAT for a 20 square metre single-storey extension, although its size examples are labelled May 2024. Treat the range as an early benchmark, not a current local quotation. Add any excluded VAT, design and approval fees, kitchen, glazing, flooring, decoration and contingency. The property, access, structural design and work required inside the existing house can move the total substantially.
Do I need planning permission for an extension in Rugby?
Not always. Some house extensions can use permitted development rights, but the property and proposal must meet every relevant limit and condition. Previous additions, planning conditions, designated land, listed status and the size, height and position of the extension can change the route. Larger rear extensions may require prior approval. Check the specific Rugby property and design, and consider a Certificate of Lawfulness when formal confirmation is important.
Do permitted development rights remove the need for building regulations approval?
No. Planning and building regulations are separate systems. An extension normally needs the relevant building regulations approval even when full planning permission is not required. The technical review can cover structure, fire safety, insulation, ventilation, drainage, electrics and glazing. Confirm who will submit the application, provide calculations, arrange inspections and collect completion paperwork. Party wall duties are a separate consideration again and may require advice before work starts.
What should an extension quotation include?
It should identify the drawings and specification used, the building work included, VAT treatment, exclusions, provisional sums, payment stages and proposed programme. Responsibility for design, structural calculations, applications, inspections, glazing, services, fittings, finishes and making good should be clear. The quotation should also explain how changes will be priced and approved. Compare several builders against the same information so that different totals describe the same finished result.
Sources
Figures, fees and rules change. Every source below was checked on 12 September 2026.
- Federation of Master Builders: Single-storey extension costsCurrent UK cost per square metre range and the main cost drivers.
- Checkatrade: House extension cost guideCurrent UK cost per square metre range, plus size examples labelled May 2024, scope assumptions and VAT basis.
- Which?: House extension costs in 2026Independent budgeting advice on quotations, VAT and contingency.
- Planning Portal: Extension planning permissionCurrent permitted development limits and conditions for houses in England.
- Rugby Borough Council: Pre-application planning adviceLocal advice route, Certificate of Lawfulness guidance and the 2026 householder pre-application fee.
- GOV.UK: Building regulations approvalWhen building regulations approval may be needed and how it differs from planning permission.
- GOV.UK: Party walls and building workWork that may require notice to a neighbouring owner.