Foundation Cost Guide – What You’ll Pay to Build a Solid Base
If you’re adding a conservatory, extending a garden room, or starting a new build, the foundation is the first big line item on your budget. It’s not just about digging a hole – you’re paying for soil prep, waterproofing, reinforcement, and the labour that gets the concrete poured right. Knowing the cost drivers up front stops nasty surprises later on.
Key Factors That Influence Foundation Costs
First, the type of foundation matters. A simple strip footing for a single‑storey extension can run between £80 and £120 per square metre, while a full‑depth raft – often required for larger structures or weak soil – usually lands in the £120‑£180 range. If you’re on a sloping site or need extra load‑bearing strength, piled foundations can jump to £200+ per square metre.
Second, ground conditions dictate extra work. Clay, sand, or high water tables may need extra excavation, soil stabilisation, or drainage systems. Those hidden costs can add £300‑£600 per day of site work, depending on the contractor and location.
Third, regional labour rates and material prices vary. Builders in South East England often charge 10‑15% more than those up north. Keep an eye on local market trends – a sudden spike in cement prices can bump your total by a few hundred pounds.
Tips to Keep Your Foundation Budget in Check
Start by getting at least three detailed quotes. Ask each contractor to break down excavation, concrete mix, reinforcement steel, waterproofing, and labour. Comparing line‑items makes it clear who’s adding hidden fees.
Consider a site survey before you commit. A geotechnical report can tell you whether a simple strip will do or if you need the expensive raft or piles. That knowledge saves you from re‑digging later.
Don’t skimp on waterproofing. A cheap membrane can lead to damp problems that cost thousands to fix. A modest increase of £5‑£10 per square metre now prevents years of headaches.
Plan your timeline wisely. If you can schedule the foundation work during a dry spell, you avoid extra de‑watering costs. Weather delays are a common budget buster.
Finally, factor in a contingency fund of 10‑15% of the total foundation estimate. Unexpected rock, a sudden change in design, or a permit tweak will almost certainly pop up.
Putting it all together, a typical 30 sqm conservatory extension in the UK might cost between £3,000 and £5,500 for the foundation alone, depending on soil, type, and location. Use these numbers as a benchmark, get solid quotes, and you’ll keep your project on track without breaking the bank.
