Extension Cost: What You Really Pay for Home Additions in the UK

When you think about adding space to your home, the extension cost, the total price to build a new room or expand an existing part of your house. Also known as a home extension, it’s one of the most common ways UK families gain extra living space without moving. But here’s the thing: most people don’t know what’s actually included in that number. Is it just bricks and mortar? Or does it cover permits, surveys, waste removal, and unexpected delays? The truth is, extension cost isn’t a single price—it’s a chain of decisions, each adding up fast.

What affects extension cost the most? It’s not just size. The building extension, a structural addition to a house that requires planning permission and skilled labour type matters—side, rear, or loft conversions each have different rules and prices. Then there’s the construction cost, the total expense of materials, labour, and equipment needed to complete a building project. In 2025, UK homeowners are seeing £1,800 to £3,000 per square metre for a basic extension, but that jumps if you need structural steel, underpinning, or high-end finishes. And don’t forget hidden bits: drainage upgrades, electrical rewiring, or connecting to gas lines can add £5,000 to £15,000 on top. Most people think they’re paying for walls and a roof, but they’re really paying for the whole system underneath.

Why do some extensions go way over budget? It’s rarely the builder’s fault. Often, it’s because the homeowner didn’t know what questions to ask before signing. Did they check if their property is in a conservation area? Did they get a structural survey before design? Did they factor in the cost of living elsewhere during work? These aren’t optional extras—they’re part of the real extension cost. The posts below show real cases: one family spent £45,000 on a rear extension and saved £12,000 by doing the plumbing themselves. Another paid £80,000 because they skipped the soil test and had to underpin the foundation. You don’t need to be an expert to avoid these mistakes—you just need to know what to look for.

Below, you’ll find real stories from UK homeowners who’ve been through it. No fluff. No marketing spin. Just what they paid, what surprised them, and what they’d do differently. Whether you’re thinking about a small kitchen extension or a full two-storey addition, the answers are here—not as guesses, but as lived experience.