A well-planned extension can improve how a house works—adding space where it’s needed, more light, or simply a better connection to the garden. It might be a new kitchen, a small studio out back, or something larger. But after the structure goes up, there’s one issue that sometimes doesn’t get enough attention: moisture.
The UK’s climate doesn’t make it easy. Rain is common across all seasons, and water has a way of finding weak points. If it isn’t dealt with early, it can slowly cause damage—starting with surface issues like stains or flaking paint, and later leading to deeper problems with walls or flooring.
While the focus during a build often leans toward design choices—tile patterns, window placement, colour schemes—keeping water out is something that needs to be factored in from the beginning. It’s not just about patching things later. It’s about planning for where water might go, and putting simple systems in place to move it away from the building before it becomes a concern.
Watch Where the Water Goes
Before laying a foundation or pouring concrete, it’s useful to take note of the land. Does water collect in one spot after heavy rain? Are the gutters already struggling with overflow?
Small changes to the landscape—like adjusting slopes or adding drains—can steer water away from the building. Downpipes should send water somewhere useful, not just onto already saturated soil. Even basic gravel-filled trenches or open channels can stop water from creeping toward the walls.
Often, these are the parts that don’t get discussed in planning meetings, but they should. Because once an extension is built, fixing drainage is harder and usually more expensive.
Don’t Forget What’s Below Ground
While surface runoff is easy to spot, water coming from the ground itself is a quieter problem. Some areas hold moisture for long periods, especially if the soil is clay-heavy or flat. Over time, that moisture can build up around the base of the extension and start to push inward.
That’s where something like an energy-efficient sump pump makes a difference. Installed in a small pit—typically below the floor—it collects water before it rises high enough to cause trouble. The pump switches on automatically and moves the water somewhere it won’t do harm, such as a storm drain or soakaway. Since modern versions only run when needed, they use very little electricity. In damp-prone gardens or below-ground extensions, this one tool can quietly protect your entire space.
Choose Finishes That Can Handle Damp
Even with good drainage, it’s smart to assume that water might find its way in eventually—whether from condensation, a plumbing issue, or just time.
Flooring that holds up under humid conditions is worth the investment. Think about materials like sealed tile, waterproof vinyl, or even concrete that’s been properly treated. Timber can work, but it needs sealing and regular checks. The same goes for walls—using moisture-resistant boards or coatings in certain areas can delay or even prevent visible damage.
Flat roofs are another concern. If yours has one, pay extra attention to the membrane and any joins or seams, especially where new walls meet existing ones. Water likes corners.
Let the Air Move
A well-sealed space is great for warmth, but not always for moisture control. Even normal day-to-day life—cooking, laundry, people coming and going—adds moisture to the air. Without somewhere to go, it settles on cool surfaces and builds up over time.
Openable windows, trickle vents, or even a basic extractor fan can make a big difference. If you’re running into musty smells or foggy glass regularly, a dehumidifier may help. The idea is simple: keep air moving so that moisture doesn’t get the chance to settle in.
Check In From Time to Time
Once your extension is done, don’t assume it’s set for life. Even the best systems need a little attention.
Clear gutters every few months. Check that drains still flow. Look for peeling paint near the floor, or damp spots that seem out of place. If there’s a sump pump in place, test it now and then by pouring in a bucket of water—it should switch on without issue. A few minutes of checking can save you days of repair work later.
A Bit of Planning Goes a Long Way
Most people build extensions for comfort—more room to live, work, cook, relax. Water damage isn’t part of that plan, and with the right steps, it doesn’t have to be.
Start with smart drainage. Use materials that last. Consider backup systems, like a sump pump, for when the weather does its worst. And don’t forget the simple things—airflow, regular checks, and being aware of how your home responds to rain.
You don’t need to over-engineer. You just need to be ready.