House Extensions: Costs, Types, and What You Need to Know Before You Build

When you talk about house extensions, additions built onto an existing home to increase usable space without moving. Also known as home extensions, they’re one of the most common ways UK homeowners gain extra room—whether it’s a bigger kitchen, a new bedroom, or a sunroom that actually gets sunlight. Unlike full rebuilds, extensions let you keep the character of your house while adding value, but they’re not as simple as picking a design and calling a builder.

The real question isn’t just can you extend—it’s how you should extend. Most people start by asking if it’s cheaper to build up, adding a second storey to an existing structure or to expand out, adding space sideways or to the back. The answer depends on your plot, your roof structure, and local planning rules. In many UK towns, building up is easier to get permission for because it doesn’t eat into your garden. But if your foundation isn’t strong enough, you’re looking at expensive structural work before you even lay a single brick.

Then there’s the cost. A simple single-storey extension might start around £25,000, but that’s before you factor in the hidden stuff—like new foundations, drainage changes, or upgrading your electrical system to handle more circuits. The most expensive part? Often it’s the structural work, the hidden framework that supports the new space, including beams, load-bearing walls, and foundation reinforcement. You can’t see it, but if it’s done wrong, your whole house could shift. That’s why so many guides stress checking your builder’s track record with similar projects—not just their portfolio of kitchens and bathrooms.

Planning permission is another minefield. Not every extension needs it, but if you’re in a conservation area, near a boundary, or your house is listed, you’re already in tricky territory. And even if you’re covered under permitted development rights, there are strict limits on height, materials, and how close you can build to your neighbor’s fence. A lot of people get caught out thinking they’re safe because their neighbor did something similar—until the council sends a notice.

And let’s not forget the timing. Most house extensions take 3 to 6 months, but delays from weather, material shortages, or neighbor complaints can stretch that to nine. The best projects plan for the worst-case scenario—not just the ideal timeline.

What you’ll find below is a collection of real-world guides that cut through the noise. From how much a 2025 UK extension actually costs, to why structural work eats up half your budget, to whether building up is truly cheaper than expanding out. You’ll see what separates a smart extension from a money pit, and how to spot a contractor who knows their load-bearing walls from their lintels. No fluff. No hype. Just what works—and what doesn’t—when you’re adding space to your home.