Retrospective claims

I am a small practitioner with a number of clients who own commercial property as investments. I am aware that there has been a redefinition of what items of plant within commercial buildings may qualify under the heading of ‘integral features’.

The question which arises is to what extent can such allowances be claimed now in the case of a business which has not previously claimed capital allowances on its integral features or indeed other items of plant where buildings were owned prior to April 2008?

For example if no previous claim has been made for capital allowances by my client would it now be possible to make a claim for such integral features as redefined from April 2008 under the heading of the annual investment allowance or does this only apply to expenditure actually incurred on or after 1 April or 6 April 2008...