We weren’t planning to release our annual cost update until the new year, but with construction prices dominating conversations among homeowners, contractors and architects alike, it felt sensible to bring this one forward. Clear cost expectations are essential to successful project planning—so here is our latest, experience-backed overview.
At CROE Architects, our figures are drawn from over 700 domestic projects across the South East—spanning Hertfordshire, the Chilterns, Oxford, the Cotswolds and Cheltenham. These aren’t theoretical numbers; they reflect what our clients are actually paying in today’s market.
1. The Reality: It’s No Longer Cheap to Build
Let’s address the obvious point first: construction is significantly more expensive than it was pre-Brexit.
10 years ago, new-build residential extensions typically fell between £1,500–£1,800 per m².
Today, we consistently see costs of £3,000–£3,500 per m² for well-built extensions using reputable contractors, with costs higher for projects located in London.
These rates generally apply to the new areas of construction only. Many homeowners assume these figures include upgrades to the existing property—however, they usually don’t.
2. Additional Costs Homeowners Often Overlook
When planning an extension, changes to the existing house can add significantly to the overall budget. Typical examples include:
Window replacements
- uPVC windows: £800 per unit installed.
Flooring upgrades
- Mid-range supply & install: £110 per m².
Bathroom refurbishments
- Modest bathroom renovation: £5,000–£7,000, depending on fittings and finishes.
Kitchen installations
- Basic kitchen: from under £5,000.
- Quality, durable kitchens: £20,000+.
Additions like these are often triggered by the extension itself—for example, aligning finishes across an enlarged open-plan space, improving energy efficiency, or modernising areas that suddenly sit adjacent to new work.
3. The Importance of Investing in the Pre-Construction Stage
One of the most effective ways to avoid unexpected costs is to invest properly before construction begins.
Clear, coordinated information—drawings, specifications, and details—from all designers involved is essential. This means:
- The Architect.
- Structural engineers.
- Landscape designers.
- Kitchen and bathroom designers.
- Any specialist consultants.
Comprehensive information ensures every element dovetails neatly together and reduces the chance of discovering costly surprises during construction.
Bring in a Quantity Surveyor Early (If Budget Allows)
We always recommend involving a QS at two key stages:
- Early design – to confirm the proposal aligns with the client’s budget.
- Detailed design – to allow value engineering before tender.
This process ensures financial clarity and prevents the disappointment of discovering late in the process that a scheme is unaffordable.
4. Planning for the Unknown
Even with the best preparation, existing buildings can present hidden challenges.
We’ve encountered properties that were very poorly constructed, with structural or fabric issues that only reveal themselves once work begins.
These situations cannot be ignored or patched over—they must be corrected. This can lead to increased costs or the need to value-engineer other parts of the project to stay within budget. Including a sensible contingency sum is therefore essential.
5. Decision-Making, Compromise and Clear Priorities
Every project involves decisions, and sometimes tough compromises. Setting clear priorities early will make this process far smoother.
We encourage clients to communicate:
- Non-negotiables (“red lines”).
- Areas where compromise is acceptable.
- Elements where alternatives may be considered.
Having this clarity helps your design team make informed decisions on your behalf and reduces stress should urgent choices arise during construction.
Ideally, a well-prepared and fully coordinated pre-construction package means snap decisions during the build will be rare. But when they do arise, clarity on your objectives prevents delays and protects the project’s integrity.
6. The Risks of Choosing the “Cheap Builder”
Some clients consider using contractors who offer substantially lower-than-average quotes. While tempting, our experience—and that of many clients—shows the risks:
- Missing items in the quote lead to cost increases during construction.
- Poor workmanship results in defects that require rebuilding.
- Contractors going bust leaves homeowners or insurers paying for repairs.
A “cheap” build often turns out to be the most expensive route in the long run.
7. The Key Takeaway
A domestic extension today is a significant investment, and costs have risen sharply in recent years. But with careful preparation, thorough pre-construction work, coordinated design information, and reliable contractors, clients can create high-quality spaces that add genuine value to their homes.
8. Treat Your Home as the Investment It Truly Is
With construction costs at an all-time high—and with prices only continuing to rise—today’s projects represent a significant financial investment.
Having the right professionals involved at every stage is essential, not only to protect that investment but to ensure the long-term wellbeing, safety and comfort of you and your home.
We wouldn’t allow an inexperienced financial adviser to manage our pensions or future when dealing with sums over £100k, so why would we expect inexperienced builders or consultants to deliver a complex extension project?
Your home deserves the same level of care, expertise and safeguarding.
At CROE Architects, we guide clients through this journey from the earliest stages—helping align design ambition with budget, manage risk, and ensure the project is delivered without unwelcome surprises.
If you’re considering a project in 2025-26 and would like tailored advice, we’re always happy to help.