Cost Advice Needed for Floor and Concrete Slab vs Block and Beam

[Dankat]

I'm after some advice from some experts out there.

We are looking to have a single storey rear extension completed on a semi-detached house - 5.8 metres across and 4 metres deep.

When choosing the floor construction, is there a simple way of working out the relative cost of concrete slab vs block & beam? Are there any websites that allow you to calculate a price per square metre for the two methods? Would there be any variance in the foundations used for each method?

Site access is via a path down the side of the house approx. 90 cms wide. Property is in South Central England.

As an aside, the soil is clay based and there is a large oak tree that will sit around 15 metres away from the extension.

Any thoughts would be appreciated.