Right,

Assuming that you dont want any cracks later on in the garages life you may want to excavted the ground to put a sub base down for the concrete to sit on, once the ground, and the fill have been compacted, then pouring the concrete with maybe a layer of mesh reinforcement within.

You may want to get a tamped finish on the concrete once it has been poured (looks slightly "ribbed" and maybe trowleed edges too.

Excavate ground under area of garage assuming you want a well built base = 4m x 6m x 0.150dp = 3.6m³, disposal of excavated material, say £20/m³ = £72.00

Concrete 4m x 6m x 0.150 deep = 3.6m³
Conc = £79.00/m³ x 3.6m³ = £284.40
Hardcore fill under garage 3.6m³ x £45/m³ = £162.00

Formwork (supporting sides of concrete while it "goes off") £25 - £35

Not sure what a local builder may want for labour, profit etc, they can usually charge pretty much what they want.

You may be ok with the planning, depends what the above link to the planning site says.


Hope this helps a bit