I plan to build a summerhouse. What should I use for foundations?

It's a half-oval / elongated D shape, about 12' x 8', and I need a
dwarf brickwork wall about 2' high. Between the walls will be paving
slabs. Ground is flat, well-drained sandy soil. The structure on top
will only be light, maybe 350lbs.

Should I run the slabs _between_ the walls? Or simply slab an oval
and then site the walls on top?

Thanks