Garden room/glorified shed - general question about foundations

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Pallet Fancier

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Hi all,
I have a gently sloping garden, maybe 1 in 7 at the steepest, running down to a water course. The ground is topsoil over clay and full of tree roots. What would be the preferred foundation for a wood frame structure?

I've thought about concrete piers, brick piles, a perimeter foundation of either brick or cement, or would a shallow raft work in these conditions?

I would like a 3 metre tall building, ideally, but my neighbour is opportunistic pond life who is always looking for ways to alleviate their boredom and feel important, and I don't want them being consulted as part of getting planning permission. Locating the build 2 metres away from the boundary would be difficult, but I would consider it, and would probably make it longer and thinner, rather than shorter and squarer, so it doesn't block the width of the garden. Would the extra height and different layout make a significant difference to the type of foundation preferred?

The watercourse has flooded that area in the past, to a depth of three to four inches. This last occurred many years ago, but in recent years the thin trickle of a stream becomes a storm drain every winter. On top of that, the council recently left a felled tree across a section of it (and will not come back to shift it) which is creating a dam, so occasional flooding might return in the future.

EDIT: just found this in Brdy's thread about his workshop build:

I have just had another look into the height restrictions and came across a handy article that states the measurement is taken from the heighest part of the ground immediately adjacent to the front of the building. This is very interesting as I thought it was measured from the lowest part of my whole garden.

This made me wonder about using the gradual slope of the garden to dig the foundations down a bit so they are "recessed" into the slope, and then build up a waterproof box, or lay a concrete floor that can drain, for the first 5 or 6 inches. Stupid idea?
 
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In short Yes, you will not stop water ingress if you build the room into the ground, just have a look on here of how many people come back again and again with water ingress problems.
This is how I solved a similar problem, no stream though, building two storey and never moved or leaked in about twenty years now.

IMGP1697 (1).jpeg

IMGP1721 (1).jpeg
 
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