You soon find out that the Civil Servants who answer the phone do little more than you'd do yourself - they look it up on the internet, too! As said above, the quickest way is to contact your own vet (who will probably end up doing the inspection on behalf of Animal Health).
Here are the broad guidelines, as I was given them when I was researching the subject for the Haynes Pig Manual and the July issue of Country Smallholding:
Getting an approved isolation unit
It doesn't cost anything to have an existing building or outdoor area approved. All you have to do is contact your local Animal Health office and an inspection will normally be carried out by a local vet, who will check that certain criteria are satisfied. These criteria include:
• The unit must only ever be used for isolation
• It should ideally be a building separate from any other housed livestock
• The floors and walls must be in good condition and easily washed and disinfected
• Disinfectant footbaths must be provided at the entrance
• Staff must use dedicated protective clothing, and protective clothing should be provided for visitors
• No other animals should come into contact with manure or effluent from the building
Outdoor isolation units are possible but:
• Paddocks must only ever be used for isolation
• They must be physically separate from any land or buildings used for other livestock
• There must be a minimum distance of 3 metres between the perimeter of the isolation paddocks and any other livestock.