gas pipe type >size that can go in the floor of are house

Your gas safe engineer will need to calculate the correct size of pipe needed for the boiler and other appliances coming off this supply and the structure off the floor that the pipe is going into Trac pipe would be the best way less joints and pre sleeved

First you don not state the type of floor the new gas service will be run in, if its a solid floor then ducted TRAC pipe would be ideal, important to remember there must be NO mechanical joints within the solid floor, only TRAC pipe, however if it is a normal wooden floor void then copper tube soldered throughout its entire length would suffice.