UFH on suspended floor or solid floor
This is going to be in a new extension, so would I better just to go for a solid floor to give a bigger thermal mass and it would appear to me(as a layman) an easier job with more insulation.
But the maths is slightly different for the floor with UFH as you have a dominant downwards heat flow because the it has a high delta T than the upwards flow. Subsoil temp stabilises at approximately 8 to 9 degrees below the floor and the ufh water flow is circa 30 degrees, so the delta T becomes 21 instead of 13 (used in the linked maths.
Heat Flow Q = U*A*dT
Heat Loss E_heat = time*average_heat_flow = (days*24hours/day) * U * A * (Average_dT)
For me average dT across 6 m