Well Im trying to help and give you the best advice i can, but is seems to be going nowhere..... Domestic properties come under part 6, and is classed as a living dwelling, a garage linked to the property would be considered the same. Any building seperated can not be classed as a dwelling. its now a building. So will come under part 1.....Lofts in dwelling are not protected under assessments due to being low risk, if any heating or ventalation equipment were to be fitted into the loft, then detection would be requested. New council builds are fitting solar inverters and water heaters in lofts at present, so detection is now becoming more common in loft areas.............Your out building can only be classed as a independant building not a dwelling, garage or commercial premises...so part 1 will need to be in place here, again a risk assessment would need to be done to advise what is needed or at least fit a P1/L1 systems to cover....The main things you need to consider her is live animals, fuels, foods, construction of building wood/brick, the tractor itself...these are all risks of a protential fire. as suggested would fire fire control panel cheep 2 zone, with siren outside premises, and then consider what detection is going to suit this enviroment