Revenue have issued Tax Briefing No. 1/2010 which deals with the VAT treatment of deeds of variation to leases. The briefing sets out by way of example the VAT treatment that applies when the substance of existing lease agreements are varied.
In particular the brief looks at the VAT implications of varying the term or length of the lease, reducing the rent in return for removal of a break clause, the partial surrender of a lease, and adding more area to the lease. The briefing including the examples can be accessed here.