I sat back on answering this because I'm not sure and I thought somebody else would be able to give you more reliable information. However nobody else has so I'll have a go but take this as my 'understanding' rather than hard fact.
I think that if it's a new building it must have wheelchair access
If it's an old building but is acting as 'an arm of the government' (ie taking clients paid for with public money) then it must also have wheelchair access.
But, as I said, I'm not sure - I just seem to remember someone saying that to me once.
Hope that helps.
Cheers,
Stuart