I have a 2014 installment agreement. Can I add what I owe from my 2015 taxes to my installment agreement from 2014?

[HollyP]

Yes, you can include the new balance (tax liability) in your current installment agreement.  There will only be one installment agreement which includes all of the tax years that you owe. 

You should contact the IRS as soon as possible to have it included because a new balance will automatically default your current agreement (it is one of the terms and conditions.) You can call them at 1-800-829-7650 or go to a local office. Or, you can submit another Form 9465, and include all of the tax years that you owe with the request to have the new tax year included. It is important to have the new tax year included because collection will start on the new tax year, if it is not added to the agreement. 

See FAQ below for more info: https://ttlc.intuit.com/replies/4678634