Skip to content
You have a number in your head. Maybe you have used an online calculator, or a friend told you what they paid three years ago.
But here is the hard truth: The “build cost” is rarely the final cost.
When homeowners run over budget, it is rarely because of the bricks and mortar. It is because of the “boring stuff”—the fees, the groundworks, and the finishing touches that often get left off the initial quote.
To help you plan a budget that actually works, here are the hidden costs that tend to catch people off-guard.
- The “Before You Build” Fees
Before a shovel hits the ground, you will need to spend money on paperwork and professionals.
- The Drawings: Whether you use an architect or a streamlined service like ours, you need professional plans.
- The Structural Engineer: Your builder cannot guess what size steel beam you need. An engineer must calculate this to satisfy Building Control.
- The Planning Fee: Even if you use Permitted Development, we strongly recommend a Lawful Development Certificate to prove the build is legal.
- The Party Wall Surveyor: This is the big one people forget. If you are digging near a neighbour’s wall (which is almost guaranteed with a terrace or semi), you need to serve notice. If they dissent, you might have to pay for a surveyor—or even two—which can add £1,000+ to the bill out of nowhere.
- The “Invisible” Money Pit: The Ground
You cannot see what is under your garden, and that makes it a financial risk.
- Soil & Roots: If your soil is poor (e.g., heavy clay) or there are trees nearby, you might need much deeper foundations than standard.
- Drains: If a public sewer runs under your garden, you will need a “Build Over Agreement” with the water company and potentially expensive lintels to bridge over the pipes.
- The “Glazing Tax”
We all love the look of floor-to-ceiling glass and massive bi-fold doors. But glass is expensive.
- Bi-folds & Sliders: Expect to pay £800–£1,200 per metre of width.
- Skylights: A simple Velux is affordable; a massive “architectural” roof lantern can cost thousands.
- Tip: Standard-sized doors are much cheaper than bespoke sizes. Design your opening to fit the doors, not the other way around.
- The “Shell” vs. The “Home”
Builders often quote for the “shell”—the walls, roof, and floor slab. They might not include the things that actually make the room liveable.
- Heating: A bigger house needs a bigger boiler. Your old boiler might not cope with the extra radiators or underfloor heating, forcing an expensive upgrade.
- The Finish: Plastering, painting, tiling, and final flooring are often excluded from the main quote.
- The Garden: Construction will wreck your patio. You need to budget for landscaping to fix the mess left outside the new doors.
- The Contingency Fund (Do Not Skip This)
It is tempting to look at the total and think, “If I’m careful, I can squeeze it all in.”
Construction is unpredictable. Prices fluctuate, pipes burst, and you might change your mind about the tile colour halfway through. Industry standard is to set aside 10–15% of your total budget as a safety net. If you do not use it, great—buy a nicer sofa. But you need it to be there.
Conclusion: Forewarned is Forearmed
Budgeting for an extension is not just about the price per square metre. It is about looking at the whole picture—from the first sketch to the final coat of paint.
Want a clearer picture of the costs? At Plan My Extension, we do not just draw lines on a page; we help you understand the realities of your build. Get a quote for your project drawings today and start your journey with your eyes wide open.
