Second Storey Extension Cost UK: What You Really Pay in 2025

When you add a second storey extension, a vertical addition to an existing single-storey home that creates a full new floor. Also known as two-storey extension, it’s one of the most effective ways to gain space without expanding your footprint. Unlike a loft conversion, this isn’t just turning an attic into a bedroom—it’s building a whole new level on top, with its own floor, walls, roof, and often new stairs. This means more disruption, more structural work, and yes, a bigger price tag.

What makes the second storey extension, a vertical addition to an existing single-storey home that creates a full new floor. Also known as two-storey extension, it’s one of the most effective ways to gain space without expanding your footprint so expensive? It’s not just the bricks and mortar. The real cost comes from structural work, the reinforcement or rebuilding of load-bearing walls, foundations, and floors to support added weight. You’re adding thousands of kilograms to an existing house, so the foundations often need strengthening—or even completely rebuilt. That’s not a DIY job. It requires structural engineers, surveyors, and certified builders. Then there’s UK building regulations, the legal standards that govern safety, insulation, fire resistance, and accessibility in home construction. These aren’t suggestions—they’re mandatory. Get them wrong, and you could face fines, forced demolition, or trouble selling later.

Compare that to a loft conversion, a renovation that turns an unused attic space into a livable room without adding new structure. Loft conversions are cheaper because they use existing walls and roof structure. A second storey extension? You’re building from the ground up—literally. That’s why prices range from £70,000 to over £150,000 depending on size, location, and finishes. London? Expect the top end. Rural areas? Maybe a bit less, but not much. Materials matter too. Steel frames cost more upfront but save time. Brick and block are cheaper but take longer. And don’t forget the hidden stuff: temporary relocation, planning permission fees, utility relocations, and the cost of living through months of dust and noise.

Most people think the cost is just about square footage. It’s not. It’s about complexity. A simple box extension with basic finishes is one thing. Add a bathroom, a staircase that fits, proper insulation to meet 2025 energy rules, and smart lighting? The price jumps fast. And if your house is on a tight plot, or near a neighbour’s property, you’ll need party wall agreements—which can delay things and add legal fees. This isn’t like buying a new kitchen. This is rebuilding part of your home’s skeleton.

What you’ll find in the posts below are real breakdowns of what drives these costs, what you can do to save money without cutting corners, and how to avoid the most common mistakes homeowners make when planning a second storey. You’ll see price tables from actual UK projects, learn which parts of the build eat up the budget, and get clear advice on when to hire a structural engineer versus when you can skip it. No theory. No guesswork. Just what works—and what costs way more than it should.