Rent from income tax department: retrospective increase

A Trust has rented out a property to the Income Tax Department. The rent was Rs.50000/- per month i.e. per annum Rent is Rs.6 lacs for which we need not register for Service Tax.

Now, for the 3 year period June 2010 to June 2013 Rent has been increased to Rs.125000/- per month which means Rs.15 lacs for the year. So the Trust will get the difference of Rs.9 lacs per year for 3 years i.e. Rs.27lacs plus Service Tax. When is the overall treatment under Service Tax and its liability?