Completely free — build up any element and check it against the targets.
Runs in your browser. Build up the element and watch it traffic-light against the targets.
There's no single "good" U-value
The U-value you need depends on the element and the compliance route. Part L 2021 sets a limiting (worst-allowed) value and a lower notional value for walls, roofs, floors, windows and doors, and the Future Homes Standard tightens them. Roofs are held to the tightest targets, floors a little higher, walls higher still — so it's judged element by element, not as one figure.
Don't memorise the numbers — check the build-up
Because the targets shift with the regs and the route, the reliable way to answer "what U-value do I need" is to build up the actual element and see where it lands. U-Monkey traffic-lights your result live against the Part L 2021 limiting and notional values and the Future Homes Standard targets, so you know whether it passes and by how much.
Check any element
Build up a wall, roof, flat roof or floor from real UK products, read the U-value and the full working, and export a Building-Control-ready PDF — all completely free.
Frequently asked questions
What U-value do I need for Building Regs?
It depends on the element and on which route you're following. Part L 2021 sets a 'limiting' (worst-allowed) U-value and a lower 'notional' value for each element — wall, roof, floor, windows and doors — and the Future Homes Standard tightens them further. Rather than memorising the numbers, U-Monkey traffic-lights your build-up against all of them live as you work.
Are the targets the same for every element?
No. Roofs are held to the tightest U-values, floors are a little higher and walls higher still, with windows and doors on their own scale. That's why there's no single 'good' U-value — it's judged element by element, which is exactly how U-Monkey shows it.
Do new builds and extensions have different targets?
Broadly, new dwellings are designed to the notional (tighter) values, while extensions and replacement elements have their own limiting standards. U-Monkey shows the result against the relevant Part L and Future Homes Standard figures so you can see where a build-up lands.
Is the calculator free?
Yes — U-Monkey is completely free, including the Building-Control-ready PDF. Build up any element and check it against the targets at no cost.
Ready? Open the U-value calculator and build up your element.