Raft foundation with a downstand beam - What does this mean?

Hi all - I live in a clay soil area in Essex with a willow tree some 3.5 m away.

I'm building a 6X5 m (30m2) outbuilding at the end of garden. Building Regs is saying I need a "raft foundation with a downstand beam to the perimeter."

Can someone please explain precisely what this means? How deep does it have to be, and what the downside beam part means?

Any diagrams or pics would be much appreciated.

Many thanks for your help.[/b]

It's just a slab thickened around the perimeter. Also called a ring beam

It normally needs designing though, and not some random dimension!

A raft spreads the building load across the entire footprint. The edge thickening is more for protection against frost heave and also gives the perimeter the ability to span over soft spots, if necessary.

The clay soil will shrink and expand with changes in its moisture content; the willow will extract moisture and exacerbate levels of shrinkage of the soil. If it's close to one side, it follows that the greatest shrinkage will occur closest to the tree.

Bear in mind that the use of a raft on clay soils will not prevent movement occurring: it merely ensures that the whole thing can move bodily, but not result in fractures to the superstructure. Thus, it can still end up on the píss, just with no cracks. Is that an improvement? Only you can decide...

THanks.