The Brew Cabin

I'm in early stages of planning for a new patio to replace old grey concrete slabbed area of the garden. I'm thinking sandstone flags. Also, a raised bed surround, formed by newly laid dryslate effect walls built about 18 inches from the garden perimiter walls.
Questions - (1) what criteria should I use to decide whether to lay on top of existing slabs. (2) if ok the leave old slabs, do I use a lesser thickness of mortar/sand and (3) should I build the new dry-slate walls on top of old slabs, new flags, and must I dige a spearate foundation for thses, and build them first before laying flags.
Thanks - and great site - I'm in the (web) business and this is one of the best I've seen.

1 - the most important factor is the level of the new paving in relation to the dpc. Will you be able to maintain a minimum of 150mm below dpc? After that, you need to consider the condition of the existing flagged surface and whether paving over the top of it is likely to cause any long term problems.

2 - the bedding layer should be at least 25mm thick. Now, if you're using sandstone flags, you'll probably find that you have a bed of variable thickness to accommodate the difference in thickness of the sandstone flags, but, in all cases, 25mm should be the minimum to aim for.

3 - Depends on the height of the walls and the condistion of the flags, as noted in (1) but I always prefer a separate foundation for walls, even dwarf walls on gardens. I know some contractors and certain publications advocate building dwarf walls on top of paving, but that assumes that the paving is sound and will not move at all during the life of the wall - how does that tie-in with your plans?