What is a good depth for solid concrete floor under screed?

Hi,

Our kitchen floor has a concrete pad under the tiles we just removed. It's about 20-25mm depth, and then is on soil (no dpm). We want to screed on top to put lino down. Is this depth of concrete OK or is it bit thin, or would we be best digging out and putting new concrete slab with dpm and/or insulation? The tiled floor was there OK, with no obvious damp probs for 110 yers. The builders are quite surprised how thin the concrete is.
Thanks,

thats to thin, you need to dig deeper and lay dpm, insulation and concret/screed base.

What has your builder adviced?

as matty says, too thin.

it is fairly typical for us to remove 17" of material (spoil) from the 'oversite' area, in order to accommodate all the necessary materials.