rate of gst

If the supplier sell the constructed units with cost of land included .......... GST rate... 12%

Otherwise.. almost all other services related to real estate fall  under 18% slab rate.

The supply of construction service has been made taxable under the GST @ 18%. However a deduction of 1/3rd of the gross value is made available in case where the value of such supply is inclusive of value of land. Hence effective value of taxable supply becomes 2/3rd i.e.,
18% * 2/3rd= 12%

So the same is taxable @ 12%

however please ensure whether u r asking for service provider of construction service as a builder or Real estate service provider , because the supply of Real Estate service is taxed at 18%