"Might be an idea to enquire if the seller would provide 2 contracts on the sale one for the land under the threshold amount mentioned above and another for the house, thus limiting stamp duty payable. "
This is simply not a runner. You cannot have two related contracts for the same property where the intention is to avoid?evade? stamp duty. It is illegal.
A site is deemed non residential property - stamp duty is payable on the disposal of a site on a sliding scale - 1% -8% on sites from E10K to 150K. Over 150K, its at 9%.
In OP's scenario, depending on the size of the plot - Revenue could deem it to be the sale of a house with surrounding curtilage ( gardens) PLUS non residential lands in addition. Stamp Duty would be payable on the house at Residential Stamp Duty rates and any additional lands ( in excess of gardens etc.,etc) would attract non residential stamp duty rates.
mf