Existing natural gas supply probably won’t cope with two boilers ,run new off LPG bottles.
Probably could be connected to existing ,you would want the new building to have its own control
Run it on its own combi boiler using LPG ,Electric heating .or yo can buy pipes inside insulated ducting, problem with last one is delivery time for hot water .
You will probably end up buying a indemnity insurance policy to cover ,that’s what I had to buy when I could not provide paperwork for my mothers property when I sold it , £100 ish