Copy of How Much Does a House Extension Cost in 2026?

As a project manager specialising in residential extensions and refurbishments and based in South East England, cost is one of the biggest worries I see for both homeowners and builders. Prices rose sharply during Covid, and while the market has stabilised, costs have not returned to pre‑pandemic levels.

When people ask, “How much does a house extension cost?”, the honest answer is that there isn’t a single figure. Costs vary widely depending on the size, complexity, location, and level of finish — and two similar‑looking projects can end up with very different budgets.

This article is intended as a high‑level summary to help homeowners understand the main cost areas involved. I’ll go into more detail on each element in future blogs.

Before any construction starts, there are a number of pre‑construction costs to allow for, including architectural fees, planning and building control fees, specialist surveys (such as ecology surveys for bats or newts), interior design, and professional project management or builder costs.

Working with an Architect or Architectural Designer

The first port of call for most homeowners is to speak to an architect or architectural designer. There are important differences between the two, and it’s worth understanding who you are working with.

Some professionals are fully qualified and registered architects, while others are architectural designers with varying levels of training and experience. Neither route is automatically right or wrong, but it is important to know who you are appointing and not be afraid to ask questions about qualifications, experience, and fees.

I would usually suggest speaking to at least three potential practices. Some architects are very busy and you may wait weeks for initial information, while others specialise in high‑end projects with fees to match. Through these early conversations, you will normally get a good sense of who you would work well with and what is appropriate for the size and specification of your project.

An architect or designer typically helps in two main ways. Firstly, they will visit your property, take measurements, and discuss what you would like to achieve. They will then advise whether your ideas are feasible and, if so, agree a fee to produce the drawings required to apply for planning permission.

Secondly, they will usually prepare and submit the planning application on your behalf, including completing the online forms and sending the drawings to the local authority. For most householder extension applications, the planning decision process typically takes around eight weeks, although this can be longer depending on the complexity of the proposal and the local council’s workload.

Planning Drawings, Surveys, and Technical Design

For planning permission drawings, fees typically range from £1,500 to £3,500, depending on the complexity of the project, the level of design input required, and where you are in the country. More detailed or design‑led schemes can sit above this, while very simple extensions may be at the lower end of the range.

In addition to drawings, local authorities often require specialist surveys as part of the planning process. These can include ecology surveys (such as bats or newts), tree surveys, flood risk assessments, or ground investigations. In my experience, councils often request these late in the process — sometimes just when you are expecting a decision.

For that reason, I would always recommend maintaining regular, polite contact with the planning officer throughout the application. Gentle follow‑ups can help avoid unnecessary delays and give you early warning if additional information is likely to be required.

Survey costs vary depending on what is requested, but a reasonable allowance is around £1,000–£1,500 per survey.

The positive news is that most residential extension applications are approved. Nationally, around 85–90% of householder planning applications are granted, and unless a proposal is particularly unusual or conflicts with local policy, extensions are generally supported.

Building Regulations Drawings and Structural Calculations

Once planning permission has been granted, the next step is usually to reconnect with your architect or designer to produce a set of Building Regulations drawings, along with structural calculations.

These drawings are far more detailed than planning drawings and are essential for two reasons. Firstly, they demonstrate compliance with building regulations. Secondly, they provide the level of information required for prospective builders to accurately price the works.

Fees for Building Regulations drawings typically range from £1,500 to £3,000, again depending on the size and complexity of the project.

Structural calculations are usually prepared by a structural engineer and will vary depending on how many walls are being removed, the size of new openings, and the type of structure involved. As a general guide, costs often range from £800 to £2,000, with more complex schemes sitting above this.

Choosing Finishes Before You Ask for Prices

Once Building Regulations drawings and structural calculations are complete, this is often the point at which homeowners are advised to contact builders and start requesting estimates. Personally, I believe this can be one of the most dangerous moments for both your budget and your future stress levels.

At this stage, there is still a huge amount that builders need to know in order to price a project accurately. Without that information, assumptions begin to creep into almost every part of the quote — and as the saying goes, assumptions rarely work in anyone’s favour.

Before seeking prices, homeowners should ideally be making decisions on the key details of the project. This includes how the heating system will work, the type of flooring, windows and doors, lighting layouts, socket locations, sanitaryware, kitchen layouts, skirting, finishes, and much more. Each of these choices has a cost implication, and without clarity, builders are forced to guess.

As a project manager, I prefer to work with clients from the very beginning of this process. I guide them through these decisions in a structured, calm way and produce a client decision schedule. This document records every agreed choice, supported by photos, descriptions, and details of where each item will be sourced.

This approach allows builders to price accurately and enables a well-planned, realistic budget. It helps avoid situations where expectations and costs drift apart — for example, allowing for high-end finishes in one area while underestimating others.

It also removes unnecessary pressure later in the build. Without early decisions, homeowners are often put in a position where a builder suddenly needs information immediately — such as choosing bathroom tiles or fittings at very short notice. This can lead to rushed decisions, limited stock choices, higher costs, and unnecessary stress. Good planning upfront avoids this entirely and keeps the project calm, controlled, and on budget.

Building Control, Appointing a Builder, and Why This Stage Matters

At this point, we can assume that your key design choices have been made and planning permission is in place. While you have been finalising these decisions, your architect or designer will usually have submitted the drawings to Building Control. This can be either a local authority building control department or an approved private inspector.

Building Control is responsible for checking that the work complies with building regulations and for carrying out site inspections at key stages of the build, before issuing final certification. Fees vary depending on the size and complexity of the project, but for a typical residential extension, costs are commonly in the region of £800 to £1,500.

With planning approved, technical drawings complete, choices clarified, and Building Control appointed, you are now in a strong position to approach builders and request prices.

I appreciate that I am biased, but this is also the stage where I would strongly recommend involving a project manager. If you are based near me, I would always suggest having a conversation so I can explain why. This point in the process often determines whether a project stays on budget or gradually becomes stressful and expensive.

When a construction company receives your drawings, they are often one of many enquiries that firm will receive that week — particularly if the company is well regarded. A good builder’s first step should be to contact you and arrange a site visit.

That visit is not just about measuring. It is an opportunity for both sides to assess whether you can work well together, for the builder to properly understand what you are trying to achieve, and to identify site-specific issues that may not be obvious from drawings alone. These can include tight access, neighbouring properties, large trees close to the house, drainage constraints, or logistical challenges that could significantly affect cost and programme.

My advice is simple: if a builder is willing to price your project purely from drawings, without visiting the site, do not use them. If they are not prepared to invest time at the outset to understand your project and win the work properly, it raises serious questions about how diligent they will be once the project is underway.

Pricing without a site visit almost always relies on assumptions, and those assumptions tend to reappear later as variations — “this wasn’t included”, “that’s extra”, or “we didn’t allow for this”. Taking the time to appoint the right team at this stage protects both your budget and your peace of mind later on.

How Builders Actually Price a Project

Having worked within construction companies, I think it is helpful to give some insight into how projects are typically priced. This will not apply to every firm, and it is not a complaint — simply information to help homeowners understand the process and make decisions with their eyes open.

In most cases, a construction company will price a project based on what they expect it will cost them to deliver the work. This is usually broken down trade by trade.

For example, consider the construction of external or internal walls. A builder will usually work on an expected cost per square metre (m²). This will include:

• Labour (for example, how many square metres of wall a bricklayer can build in a day)

• Materials such as bricks, blocks, insulation, wall ties, sand, and cement

As a very rough illustration, a builder might expect the combined labour and materials for a standard wall to cost in the region of £120–£180 per m², depending on specification and location.

They would then carry out a take-off from the drawings to calculate how many square metres of wall your project requires. If your extension required 10 m² of wall, and the internal cost was £150 per m², the base cost to the builder for that element would be £1,500.

On top of this, builders will usually include a contingency allowance — often around 10% — to cover small price fluctuations, productivity issues, weather delays, or minor inefficiencies that occur on almost every project.

Once the direct build costs are calculated, the company must then add its operating overhead and profit. This covers the real costs of running a business, including vehicles, insurance, office staff, project management, supervision, tools, and general risk. Depending on the size of the company, the region, and the type of work, this is often in the region of 30–40%.

Finally, VAT is added where applicable, which for most residential extension works is charged at 20%.

Understanding this process helps explain why prices can vary significantly between builders, and why apparently small changes in design or specification can have a large impact on the final cost.

How a Project Manager Prices the Same Work

I wanted to take a moment to explain how I would approach pricing the same wall as a project manager, as this often helps homeowners understand where real savings can be made.

Using the same example as before, the baseline cost to build the wall — covering labour and materials — would still be around £150 per m². That underlying cost does not change.

Instead of adding contingency, overhead, and profit in the way a construction company typically would, I charge a project management fee of between 20–25%. My role is to manage the labour directly, coordinate the work on site, and order materials on your behalf. All labour and materials are paid from your own account, managed transparently by me.

Because you are paying the actual cost of the work as it happens, there is no need for an artificial contingency allowance. Costs may move slightly up or down as the project progresses, but you only pay what the work genuinely costs. It is worth noting that contingencies added by construction companies are rarely refunded if they are not used.

The management margin is lower because I am not carrying the overheads of a large construction firm. My management fee includes VAT, so there is no additional VAT added on top later in the process. The saving comes from the fact that there is no VAT applied to unused contingency, and labour from individual trades is often not subject to VAT where those trades are below the VAT registration threshold. This can make a significant difference to the overall cost when compared to a traditional contractor‑led model.

There is also an important difference in how projects are managed day to day. Project managers working within construction companies are often responsible for multiple sites at once — I have known individuals managing ten or more projects simultaneously. I deliberately limit myself to no more than two projects at any one time, which allows me to spend far more time on site, coordinate trades properly, and work proactively for you rather than reactively dealing with problems.

For many homeowners, this approach results in lower overall costs, clearer visibility of where money is being spent, and a calmer, better-managed build experience.

It is also worth addressing how unforeseen issues are handled, even after meticulous planning. Occasionally, something genuinely unexpected is uncovered once work begins — for example, when scaffolding is erected and a chimney is found to be in poor condition and in need of repair.

In a traditional contractor-led model, this additional work would typically be priced in the same way as the original build elements: the cost of labour and materials, plus contingency, plus the contractor’s margin, and then VAT on top.

Because I am already managing your project, I do not add an additional management charge for these unforeseen items. I am very conscious that unexpected costs are frustrating for homeowners, and my aim is not to compound that stress. In these situations, the charge is simply the actual cost of the repair — with no added contingency, no additional margin, and no extra VAT beyond what is genuinely applicable. In many cases, this can represent a significant cost saving compared to a traditional approach.

So How Much Does the Build Itself Cost?

With all of the preparation work complete, a common and very fair question is: so how much is the extension itself likely to cost?

At the time of writing, most builders are pricing residential extensions at between £2,200 and £3,200 per square metre, depending on location, access, complexity, and specification. For two-storey extensions, this figure is usually calculated on the total combined floor area of both floors.

As an example, a 40 m² single-storey flat roof extension might typically fall in the region of £90,000 to £125,000 for the build cost alone. This is a broad guide rather than a fixed figure, and individual projects can sit outside this range.

Costs will increase where the extension includes high-value elements such as kitchens and bathrooms, or where a higher level of finish is chosen. As a general guide:

• A typical kitchen, supplied and installed, often ranges from £12,000 to £25,000, but can exceed this for more bespoke or premium options.

• A bathroom, supplied and installed, commonly ranges from £6,000 to £12,000, depending on layout, fittings, and specification.

It is also worth remembering that figures found online are often averages that do not reflect the realities of individual sites, local labour markets, or the decisions made during the design process. Two projects of the same size can vary significantly in cost based purely on specification and complexity.

Running a project with a project manager, such as myself, will typically result in an overall saving of around 15% compared to a traditional contractor-led approach. On a typical extension costing £120,000, this could represent a saving of approximately £18,000.

On a more comprehensive extension that includes a new kitchen and bathroom, where total project costs can reach £200,000, a 15% saving could equate to around £30,000. For many homeowners, that difference alone can significantly change what is achievable within their budget.

Cost savings, however, are only one part of the picture. Equally important are the increased level of attention, proactive management, and ongoing support throughout the build — all of which help reduce stress, avoid disputes, and keep projects running smoothly.

Fixtures, Fittings, and the Often-Forgotten External Works

One final area that is often overlooked when budgeting for an extension is fixtures, fittings, and external works. Items such as furniture, curtains, blinds, and soft furnishings can add up quickly and are rarely included in build costs, yet they are essential to making the new space feel finished and usable.

Another commonly forgotten element is landscaping and external works. If you are extending your home, there is a strong chance that parts of the garden will need attention afterwards to make everything work visually and practically again. This might include reinstating lawn areas, forming new planting beds, or adjusting levels around the extension.

It is also very common for extensions to require new or extended patios, paths, or steps to properly connect the house to the garden. In some cases, fencing may need to be replaced, or driveways and access routes that have been used for material storage, scaffolding, or skips may need repairs or resurfacing.

I regularly see projects where the internal spaces are completed to a very high standard, but there is little budget left to address the outside areas simply because these costs were not considered early enough in the planning phase. Allowing for these items from the outset helps ensure the whole project — inside and out — feels complete, balanced, and properly finished.

A Worked Example: Putting the Costs Together

To help pull everything together, it can be useful to look at a fictional but realistic example of a typical residential extension.

Let’s assume a 40 m² single-storey rear extension with a flat roof, including a new kitchen and one bathroom, built to a good mid-range specification.

Typical cost breakdown:

• Pre-construction costs (architect, planning, surveys, Building Control, technical design): £8,000–£15,000

• Build cost at £2,500 per m² (40 m²): £100,000

• Kitchen supplied and installed: £18,000

• Bathroom supplied and installed: £8,000

• Fixtures, fittings, and basic external works: £10,000–£15,000

This would place the total project cost in the region of £145,000–£160,000 when

delivered through a traditional contractor-led route, depending on specification and site conditions.

Using a project-managed approach, where labour and materials are paid at cost and management is charged transparently, a saving of around 15% is often achievable. On a project of this size, that could represent a saving of £20,000–£25,000, while also providing closer oversight, clearer cost visibility, and significantly reduced stress.

As with all building projects, every home is different. The purpose of this example is not to provide a fixed price, but to give a realistic framework so homeowners can plan sensibly, ask the right questions, and avoid unpleasant surprises along the way.

Conclusion

Planning and delivering a house extension is a significant undertaking, both financially and emotionally. While no two projects are the same, understanding how costs are built up — and where risks and assumptions can creep in — puts you in a far stronger position to make good decisions and avoid unnecessary stress.

With the right level of planning, clear information, and proper management, most of the common budget overruns and frustrations can be avoided. Whether you choose to work with a builder directly or adopt a project-managed approach, the key is transparency, realistic expectations, and taking the time to make informed decisions early.

If you are planning an extension or refurbishment in South East England and would like a calm, honest conversation about costs, risks, and how to approach your project, you are welcome to get in touch.

You can contact me at adampriceprojectmanagement@gmail.com or visit www.adampriceprojectmanagement.com for more information.