I think I am correct in saying that building Regs part L1a states that a house over 150 sqm, should have its heating system zoned. However the builders I work for don't want to zone it due to added cost of installation and time. I think they have stated that because the house had its planning permission granted in 1997. They are working to 1997 regs. Not sure if this means they don't have to comply with Building Regs Part L1a any thoughts on this. Thanks.