Types of Extensions: Build Up, Expand Out, and What Works in 2025

When you think about types of extensions, the different ways you can add space to a home by building outward or upward. Also known as home additions, it's not just about square footage—it’s about how you use your land, your budget, and your local planning rules. Most homeowners face this choice: do you build up with a second storey, or expand out into the garden? Each option has different costs, legal hurdles, and long-term value. And in 2025, with material prices still shifting and planning rules tightening in the UK, the answer isn’t as simple as it used to be.

One major factor is planning permission, the legal approval needed before starting any structural change to your home. For house extensions, some projects fall under permitted development rights, meaning you don’t need full planning approval—but only if you stick to strict limits on height, footprint, and materials. A second storey extension, adding a full upper level to your home, often triggers stricter rules because it changes the skyline and can impact neighbours’ light. Meanwhile, a side or rear extension that stays low and doesn’t encroach too far into the garden might fly under the radar. But here’s the catch: even if you don’t need planning permission, you still need building regulations approval to make sure the structure won’t collapse, leak, or catch fire.

Then there’s the cost. Building up usually costs more upfront because you’re reinforcing the existing foundation and structure. But you’re also gaining space without losing garden area. Expanding out might seem cheaper at first, but if you need to move drainage, reposition utilities, or rebuild a patio, the hidden costs add up fast. In 2025, UK homeowners are seeing UK house extension costs, the total price to add space to a home, including materials, labour, and fees. Also known as home addition costs, these vary wildly by region, with London and the South East running 20-30% higher than the national average. And don’t forget the disruption. A build-up project means living with scaffolding for months. An extension out might mean losing your favourite spot in the garden.

What you choose also depends on your home’s layout. A narrow terraced house in London? Building up is often the only practical option. A detached home with a big garden? Expanding out gives you more flexibility in design. And if you’re thinking about resale value, a well-designed second storey can add more value per square foot than a ground-floor extension—especially if it adds a proper bedroom or bathroom.

There’s no one-size-fits-all answer. The best extension for you depends on your budget, your land, your local council’s rules, and what you actually need—more bedrooms, a bigger kitchen, or just more light. Below, you’ll find real-world breakdowns of costs, planning rules, and common mistakes made by homeowners who rushed into the wrong kind of extension. Whether you’re weighing a loft conversion against a single-storey rear extension, or wondering if a basement extension makes sense, the articles here cut through the noise and show you exactly what’s happening in 2025.