The following buildings are exempt from Part B (Fire safety) of the Building Regulations ( see
http://www.planningportal.gov.uk/permission/responsibilities/buildingregulations/approvalneeded/exemptions):
1. Subject to paragraph 3, a greenhouse.
2. A building used, subject to paragraph 3, for agriculture, or a building principally for the keeping of animals, provided in each case that?
(a) no part of the building is used as a dwelling;
(b) no point of the building is less than one and a half times its height from any point of a building which contains sleeping accommodation; and
(c) the building is provided with a fire exit which is not more than 30 metres from any point in the building.
3. The descriptions of buildings in paragraphs 1 and 2 do not include a greenhouse or a building used for agriculture if the principal purpose for which they are used is retailing, packing or exhibiting.
4. In paragraph 2, ?agriculture? includes horticulture, fruit growing, the growing of plants for seed and fish farming.
Also, if the buildings are not exempt but are owned by the same organisation then there is probably no requirement under the Building Regulations for a minimum space separation distance (this is what NT was getting at).
If a fire in the new hay barn could compromise the means of escape from the office then there might be implications under the Fire Safety Order. A lot depends on the ownership of and responsibilities for the respective buildings. It might be, for example, that the responsible person for the office building falls foul of the Fire Safety Order while the owner of the barn has no problems under legislation. Life can be so unfair.
If the hay barn does not have the potential to compromise the means of escape from the office then legislation has little to say about the matter. That doesn't mean to say that a fire in the barn won't spread to the office (or vice versa) and the owner/manager of the office might be right to feel vulnerable if, indeed, the buildings are so close that fire might spread from one to the other. If I owned a building and a neighbour introduced a threat to my property then I would make every effort to inform the person who introduced the threat of my concerns and ask them to remedy the situation as soon as possible. Legally, this would put me in a much stronger position to sue for negligence if the worst were to happen.
If the threat was very real and obvious then the owner of the office might be able to get a court order to remove the threat, i.e. move the barn. The owner of the office would have to be able to demonstrate that there existed a real threat to his/her property and that would probably not be an easy matter to demonstrate. Even if the hay barn is right up against the office it's not a threat unless it catches fire, so if there is little chance of it catching fire then no threat has been demonstrated.
As for assessing the adequacy of the separation (and the likelihood of fire spread), you can try to use the tables in section B4 of ADB but, as Wee Brian has said, BR187 will give better results.
