Understanding House Extension Cost Per Square Metre
When you start researching house extension cost per m2, you will quickly find that figures range from £1,200 to well over £3,000 per square metre. That is an enormous range, and it can make planning feel impossible. The truth is that the cost depends on what you are building, how it is specified and where you are in the country.
This guide explains what drives the price per square metre for different types of extension and gives you realistic 2026 figures based on projects in Lancashire and the wider North West. We have built hundreds of extensions in the region, and these numbers reflect what homeowners are actually paying, not theoretical estimates.
Cost Per Square Metre by Extension Type
Different extension types have different cost profiles because of the proportion of expensive elements (foundations, roof, external walls) relative to the total floor area. Here are the current ranges for house extension cost per m2 in the North West.
Single-storey rear extension: £1,500 to £2,500 per sqm. This is the most common type of extension and usually the most cost-effective way to add space. A 20 sqm single-storey extension typically costs £30,000 to £50,000 all in. The lower end of this range assumes a straightforward build on good ground with a flat roof and standard finishes. The higher end includes a pitched roof, bi-fold or sliding doors, underfloor heating and a quality kitchen or living space fit-out.
Double-storey extension: £1,200 to £2,200 per sqm. A double-storey extension costs less per square metre than a single storey because you are sharing the foundations and roof costs across two floors. A 40 sqm double-storey extension (20 sqm on each floor) typically costs £48,000 to £88,000. This is often the best value way to add both living space and bedrooms to a property. Our house extensions service covers everything from initial design through to completion, and double-storey projects are one of our specialities.
Side return extension: £1,800 to £3,000 per sqm. Side return extensions are common in terraced and semi-detached houses where the kitchen runs alongside a narrow alleyway. They are relatively small in floor area (often just 5 to 10 sqm) but the cost per square metre is higher because the foundation and structural costs are spread over a smaller area. A typical side return extension costs £10,000 to £25,000.
Wrap-around extension: £1,600 to £2,800 per sqm. A wrap-around extension combines a rear extension with a side return, creating an L-shaped addition that significantly increases the ground floor footprint. These are popular in semi-detached houses where the kitchen is small and there is a side passage that can be built over. Costs for a typical wrap-around are £35,000 to £70,000.
Over-garage extension: £1,300 to £2,000 per sqm. Building above an existing single-storey garage can be cost-effective because the foundations are already in place. However, the existing garage foundations may need strengthening to take the additional load, which can add £3,000 to £8,000 to the project. A structural engineer will assess this early in the process.
What Affects the House Extension Cost Per M2?
Understanding why prices vary so much helps you make informed decisions about specification and budget. Here are the main factors that move the needle.
Foundations. Standard strip foundations cost £100 to £150 per linear metre. If you have clay soil, trees nearby or poor ground conditions, you may need deeper foundations or engineered pile foundations, which can cost £200 to £400 per linear metre. In Lancashire, clay soils are common in many areas, and tree roots from mature gardens can necessitate deeper foundations. This is one of the biggest variables in extension costs and often is not confirmed until the ground is opened up.
Specification and finishes. The house extension cost per m2 figures above assume mid-range finishes. If you want underfloor heating, high-quality tiling, bi-fold doors, vaulted ceilings with exposed beams, premium kitchen units or luxury bathroom fittings, the cost per square metre increases. On the other hand, a basic plastered and painted room with standard windows and carpet is at the lower end.
Structural complexity. If the extension involves removing a load-bearing wall between the existing house and the new space (which it usually does), you will need a steel beam. Simple steel beams cost £1,000 to £2,500. Larger openings or complex geometries require more expensive steelwork, sometimes £3,000 to £6,000. Some designs involve multiple steels working together, which needs careful engineering.
Roof design. A flat roof is the simplest and cheapest option. A single-pitch lean-to roof adds a modest premium. A pitched roof to match the existing house costs more because of the additional structural timber, tiles and ridge work. Vaulted or lantern ceilings are at the top end for roof costs but create stunning internal spaces.
Location and access. Building costs in the North West are generally lower than in London and the South East, which is reflected in our figures. Within the region, access to the site affects cost. Properties with good access where materials can be delivered and machinery can reach the work area are cheaper to build than properties with narrow passages, no side access or restricted parking. If you are looking for experienced builders in Preston and the surrounding area, local knowledge of ground conditions and planning requirements makes a real difference.
Planning Permission vs Permitted Development
Many single-storey rear extensions can be built under Permitted Development rights without needing a full planning application. The rules are specific and depend on the type of house, its location and the size of the extension. Key limits include a maximum depth of 3 metres for attached houses (or 4 metres under the Larger Home Extension scheme with prior approval) and 4 metres for detached houses, with a maximum height of 4 metres.
Double-storey extensions, side extensions close to boundaries and extensions in conservation areas, Areas of Outstanding Natural Beauty or on listed buildings almost always need planning permission. A planning application currently costs £206 for a householder application and takes approximately 8 weeks for a decision.
Even when planning permission is not required, you should always check with your local authority before starting work. It is worth getting a Lawful Development Certificate (£103) which provides formal confirmation that your extension is permitted development. This avoids any disputes later, particularly when you come to sell the property.
The Party Wall Act
If your extension is built on or near the boundary with a neighbouring property, the Party Wall etc. Act 1996 may apply. This requires you to serve formal notice on your neighbours before work begins. If they consent, the process is straightforward and costs nothing. If they dissent or do not respond, you will need to appoint a party wall surveyor to prepare a Party Wall Award. This typically costs £700 to £1,500 per neighbour. In some cases both parties appoint their own surveyor, which can push costs higher.
Do not skip this process. It is a legal requirement and ignoring it can lead to disputes, injunctions and significant delays. Most neighbours are perfectly reasonable about extensions as long as they are kept informed and the process is handled properly.
Building Regulations
All extensions need Building Regulations approval regardless of whether planning permission is required. Building Regulations ensure the extension is structurally sound, energy efficient and safe. Your contractor will typically manage the Building Control process, which involves inspections at key stages: foundations, damp proof course, drainage, structural steel, pre-plaster and completion.
Building Regulations fees vary by council but typically cost £400 to £900 for a domestic extension. You can use your local authority’s Building Control service or an approved private inspector.
Professional Fees
Before construction begins, you will incur professional fees for design and approvals. These include an architect or architectural designer (£1,500 to £5,000 depending on the complexity and level of service), structural engineer (£500 to £1,500 for calculations and drawings), planning application fee (£206 if needed), Building Regulations fee (£400 to £900) and a party wall surveyor (£700 to £1,500 if needed).
These fees add £3,000 to £8,000 to the total project cost and are separate from the construction house extension cost per m2 figures quoted earlier. They are essential and should not be seen as optional extras. Good design and proper engineering save money during construction and produce a better result.
Typical Timelines for House Extensions
From first enquiry to moving into your new space, here is a realistic timeline for a typical extension project. Design and planning: 8 to 16 weeks (longer if planning permission is needed). Building Regulations approval: 2 to 4 weeks. Contractor procurement and scheduling: 4 to 8 weeks. Construction for a single-storey extension: 10 to 14 weeks. Construction for a double-storey extension: 14 to 20 weeks.
The overall process from initial idea to completion typically takes 6 to 12 months. Starting the design process in autumn or winter means you can begin construction in spring, which is ideal for groundworks and external building work.
If you are planning to include a new kitchen in your extension, which is extremely common, our kitchen fitting team works alongside the build crew so the kitchen is installed as part of the project rather than as a separate contract. This is more efficient and ensures everything fits together properly.
Making the Most of Your Extension Budget
To get the best value from your house extension cost per m2, focus on getting the fundamentals right. Good foundations, proper insulation, quality windows and well-planned structural openings will serve you for decades. These are the elements you cannot change later without significant expense.
Internal finishes like flooring, decoration and even kitchen units can always be upgraded over time if budget is tight at the outset. It is far better to have a well-built extension with modest finishes than a poorly built one with expensive cosmetics.
Consider how the extension will work with your existing house. The best extensions feel like they were always part of the property. This means careful attention to ceiling heights, floor levels, sight lines and how you move between rooms. A good designer will think about all of this before a single brick is laid.
Considering an extension? Contact us today for a free consultation and realistic cost estimate. We will visit your property, discuss your requirements and explain exactly what is involved so you can make an informed decision.
