Aluminium Windows for Extensions: Planning and Design Guide

22 January 2026 | General

Let’s be honest – planning an extension is equal parts exciting and overwhelming. You’re dreaming about that extra space, the natural light flooding in, maybe finally having room for a proper dining table. But then comes the reality: Building Regulations, U-values, thermal breaks, and a hundred decisions you didn’t know you’d need to make.

Here’s the good news: getting your windows right doesn’t have to be complicated. Let’s walk through everything you need to know about aluminium windows for your extension, without the jargon overload.

Why Everyone’s Choosing Aluminium

Walk down any street with recent extensions and you’ll spot a trend – sleek, slim-framed windows that seem to disappear into the design. That’s aluminium doing its thing.

The magic is in the material itself. Aluminium is incredibly strong, which means manufacturers can create frames that are genuinely slim – sometimes just 20mm wide. Compare that to chunky uPVC frames, and you’ll understand why aluminium has become the default choice for contemporary extensions.

But here’s what really matters: those slim frames mean more glass. More glass means more light. And more light means your extension actually feels like the spacious, airy room you’ve been imagining, not a dark afterthought tacked onto your home.

The Warmth Question (Because Everyone Asks)

“But doesn’t aluminium feel cold?” It’s the first thing most people worry about, and it’s a fair question based on old technology. Here’s what’s changed: modern aluminium windows use something called thermal breaks.

Think of it as a high-performance insulator sandwiched inside the frame, separating the cold outside from the warm inside. The result? Aluminium windows that keep heat in just as well as any other material – sometimes better. We’re talking U-values around 1.0 W/m²K, which is actually excellent (lower numbers mean better insulation, in case you were wondering).

Your heating bills won’t suffer, and you definitely won’t feel drafts.

Planning That Actually Makes Sense

Before you get lost in catalogues and samples, pause and think about the basics. How will you use this space? A home office needs different window placement than a kitchen extension. A playroom has different priorities than a quiet reading nook.

Here’s a practical exercise: visit your extension site at different times of day. Where does the sun hit in the morning? Where are the best views? Where would you naturally want to look out? These observations will guide your window placement far better than any design guide.

And please, think about privacy. That stunning floor-to-ceiling window might overlook your neighbour’s patio. A simple adjustment in height or position can save awkward curtain situations later.

The Regulation Reality Check

Building Regulations aren’t trying to ruin your design dreams – they’re just making sure your extension is safe, warm, and well-ventilated. Let’s break down what actually matters:

Your windows need to keep heat in (that U-value thing we mentioned – aim for 1.6 W/m²K or better). They need to provide fresh air through trickle vents in the frames. And you’ll need enough opening windows for ventilation – about 5% of your floor area.

Most reputable aluminium systems meet these requirements as standard, so you won’t need to become a regulations expert. Your installer should handle the technical compliance; you just need to know it’s being taken care of.

Design Ideas That Actually Work

Go Big, Go Bold: One large window often works better than several small ones. It’s cleaner, lets in more light, and costs less than you’d think.

Think Corner to Corner: If your extension has a corner, consider wrapping the glass around it. Modern aluminium systems can eliminate the corner post entirely, creating this seamless indoor-outdoor feeling that’s genuinely breath-taking.

Mix Fixed and Opening: You don’t need every window to open. Large fixed panes cost less and perform better thermally, while strategically placed opening sections give you the ventilation you need.

Consider Sliding Rather Than Bifolds: Bifolds are popular, but large sliding doors often work better for everyday use. They’re smoother, simpler, and you don’t need to fold back multiple panels just to let the dog out.

The Colour Conversation

Aluminium can be powder-coated in virtually any colour you can imagine. Anthracite grey remains the most popular choice for good reason – it’s contemporary without being trendy, and it works with almost any brick colour.

But here’s a trick designers use: choose different colours for inside and outside. Maybe anthracite outside to match your existing windows, and white inside to maximise light reflection. It costs a bit more but creates a really polished look.

What Good Installation Looks Like

This matters more than the windows themselves: find an installer who specialises in aluminium. Not someone who “does a bit of everything” – an actual specialist.

They should survey your extension personally, provide detailed technical drawings, and talk you through exactly how the windows will integrate with your build. If they’re vague about timing or specifications, keep looking.

Making It Happen

The result of all this planning? An extension filled with natural light, seamlessly connected to your garden, and genuinely adding value to your home and your daily life.

At Direct Trade Windows, we guide homeowners through these decisions every day, making sure your extension windows deliver exactly what you’re hoping for.