Rate of Interest for tax due

I have an extra tax to be paid in lieu of the rent that am receiving on a property that i own, now i was wondering about two things

1) Would this mandate an ITR2 for me, if everything else is falling under ITR1 till now.

2) The tax that this income would attract would also have some interest (as the tax was not paid till March 31st), so how to calculate that extra interest.

will appreciate a response on the same.

Thanking in anticipation.