A single-storey extension can change the way a home works every day – whether that means a larger kitchen for family life, a light-filled dining area or a practical utility space that takes pressure off the rest of the house. The single storey extension cost is understandably one of the first questions homeowners ask, but a useful answer depends on more than the square metres being added.
A well-planned extension is an investment in both space and quality of life. The aim should not be to choose the lowest headline figure, but to set a realistic budget for a design, specification and build standard that will serve your home properly for years to come.
What does a single storey extension cost?
As a broad guide, a professionally built single-storey extension in Dorset, Somerset or Wiltshire may cost around £2,000 to £3,500 or more per square metre. This is a starting point rather than a fixed price. Smaller extensions can cost more per square metre because essential elements such as foundations, drainage, structural work and access are required regardless of size.
For many homeowners, a modest rear extension may begin from around £50,000 to £70,000, while a larger kitchen-diner extension with significant glazing, bespoke joinery and a high-quality finish can sit well above £100,000. The final figure will depend on the property, site conditions and the decisions made before work starts.
It is also worth separating the construction budget from the full project budget. Design fees, planning costs, structural engineering, surveys, building control fees, kitchen supply, decorating and landscaping may all need to be allowed for. A clear quotation should explain what is included and identify items that are still subject to selection or investigation.
The main factors behind single storey extension cost
The footprint matters, but it is only one part of the calculation. A straightforward rectangular extension onto a level site is generally more economical than a design with multiple corners, a difficult connection to the existing house or extensive structural alterations.
Ground conditions and foundations
Every extension starts below ground. Soil type, trees, drains, nearby structures and the depth of the existing house foundations can all affect the foundation design. In some cases, deeper excavations or specialist foundations are needed. This is why an early site visit and appropriate surveys are valuable – they reduce the risk of discovering a major cost after work is underway.
The existing property
Older homes often come with character and complications in equal measure. Uneven walls, uncertain drainage routes, outdated services and hidden defects can affect the scope. Extending a period property may also call for more considered detailing so that the new work feels right alongside the original building.
The point where the extension joins the house deserves particular attention. Opening up a rear wall may require steelwork, temporary support and careful alterations to floors, ceilings and services. It can create the open-plan room homeowners want, but it is not simply a matter of removing a wall.
Design, glazing and roof form
Large rooflights, sliding or bi-fold doors, aluminium windows and vaulted ceilings can make an extension feel exceptional. They can also add considerably to the budget. That does not mean avoiding them. It means deciding where they will have the greatest effect and balancing them with the rest of the specification.
A flat roof extension can be a clean, practical choice, especially when it allows for rooflights. A pitched roof may suit the character of the house and provide a more traditional finish. Each option has different structural, insulation and drainage requirements, so the best choice is the one that suits the property and the way the room will be used.
Services and internal finishes
The visible finish is where many budgets rise. Kitchens, worktops, appliances, flooring, lighting, underfloor heating, cabinetry and decorating vary enormously in cost. A quality extension needs a sound shell first, but the internal specification is what turns it into a space that feels complete.
Allow for electrical work from the beginning. Think about sockets, task lighting, pendant lights, external lighting, data points and extractor arrangements before walls are closed up. Making changes later is usually less efficient and can compromise the finish.
Access and site logistics
A narrow side access, limited parking or a property on a busy road can affect labour and delivery arrangements. If materials must be moved through the house or stored carefully on a restricted site, the programme and cost may change. Good planning helps keep disruption manageable for both the homeowner and neighbours.
Budget for the whole project, not just the build
A reliable budget recognises the stages that happen before and after the main construction work. Depending on the project, these may include:
- measured surveys, drawings and design development
- planning applications or lawful development certificates
- structural calculations and building regulations information
- site investigations, drainage checks and party wall matters
- kitchen, flooring, decorating, external works and landscaping
Not every extension needs every item, but leaving them out of the early conversation can create an unrealistic headline cost. VAT should also be checked carefully, as this will normally apply to residential extension work and can make a material difference to the final budget.
A sensible contingency is also worthwhile, particularly for older properties. Around 10 to 15 per cent is often appropriate where there are unknowns. It is not a licence for poor planning. It is a practical allowance that gives you room to deal with genuine discoveries without putting the entire project under pressure.
Planning permission and building regulations
Some single-storey extensions can be built under permitted development rights, subject to limits and conditions. However, this depends on the property, the location, previous alterations and the proposed design. Conservation areas, listed buildings and other planning restrictions need particular care.
Even when planning permission is not required, building regulations approval will usually still be needed. This covers structural safety, insulation, ventilation, drainage, fire safety and more. Good design brings these requirements into the project early, rather than treating them as obstacles once work has begun.
If the work affects a shared boundary or involves structural work close to a neighbouring property, party wall procedures may be relevant too. Discussing the project openly with neighbours before construction starts is often a sensible step. It helps set expectations around access, working hours and the likely programme.
How to get a more accurate extension quote
The more clearly a project is defined, the more meaningful the price will be. A builder can provide an initial guide from a conversation and basic dimensions, but a detailed quotation needs drawings, a clear scope and an understanding of the desired finish.
When comparing quotations, look beyond the total. Check whether foundations, drainage, steelwork, insulation, windows, plastering, electrics, plumbing, decorating and waste removal are included. Ask how provisional sums and client-supplied items are handled. A lower price can appear attractive until essential parts of the work are added back in later.
It is equally useful to discuss priorities honestly. Perhaps the kitchen is central to the project and deserves more of the budget, while a simpler roof design keeps costs in check. Or perhaps extra floor area matters more than premium glazing. Building your vision with our expertise means making these decisions with a clear view of the practical trade-offs.
Where quality pays off
An extension should feel like part of the home, not an add-on. Careful brick matching, tidy roof details, well-fitted windows, accurate carpentry and considered internal finishes all influence that result. They also protect the building from the weather and reduce the likelihood of expensive remedial work later.
The cheapest route is rarely the most cost-efficient one. Poor insulation, rushed drainage, inadequate ventilation or weak detailing can affect comfort, energy use and maintenance for years. A trusted local builder will help you focus spending where it counts: on a sound structure, a dry and warm envelope, and workmanship that stands up to daily life.
The best first step is a detailed conversation about how you want to live in the finished space. Once the design responds to your home, your priorities and the realities of the site, the budget becomes far easier to plan with confidence.