House Extension: Costs, Types, and What You Need to Know

When you think about a house extension, a physical addition to an existing home that increases usable space without moving. Also known as a home extension, it’s one of the most common ways homeowners in the UK and US add value, comfort, or functionality to their property. Unlike a full rebuild, a house extension works with what’s already there—saving time, reducing waste, and often staying under budget. But not all extensions are the same. Some add a second floor, others push out sideways, and a few even turn a garage into living space. The key is knowing which type fits your goals, your land, and your local rules.

One big decision you’ll face is whether to build up, adding a storey on top of your existing home or expand out, extending horizontally into your garden or yard. In the UK, building up often means less disruption to your garden and fewer neighbor complaints, but it can require stronger foundations and more complex structural work. Expanding out is usually simpler to plan, but it eats into your outdoor space and might need planning permission if it goes beyond permitted development limits. The planning permission, official approval from local authorities before construction begins rules vary by region—what’s allowed in Manchester might be blocked in London. And don’t forget: even if you don’t need full planning permission, building regulations still apply. These cover insulation, fire safety, structural strength, and drainage. Skipping them can cost you more later when you sell.

The most expensive part of any house extension? It’s rarely the bricks and mortar. It’s the structural work, the hidden framework that supports the new space, including beams, foundations, and load-bearing walls. If you’re adding upstairs, you might need to reinforce the ground floor. If you’re extending sideways, you may need to dig deep for new footings. That’s where most budgets blow out. A simple single-story extension can cost £30,000–£60,000 in the UK, while a two-story version often hits £70,000–£120,000. In the US, prices swing wildly by state—Massachusetts and California are among the priciest. But the real savings come from planning ahead. Get multiple quotes. Ask contractors how they handle surprises. And never skip a structural engineer’s sign-off.

What you’ll find in the articles below are real breakdowns of what works—and what doesn’t. From cost comparisons between building up and expanding out, to how to spot if your foundation can handle the extra weight, to what permits actually matter in 2025. No fluff. Just straight talk from people who’ve done it, messed it up, and fixed it. Whether you’re in a terraced house in Birmingham or a suburban home in Massachusetts, this collection gives you the facts to make a smart move.