I think there is a distinction to to made here. If you owe money on your 2014 taxes, it is due by April 30th, 2015. If you make several payments over the next few months to pay that off, those are late payments and, yes, you will have to pay some interest on it.

That is not what they mean when they talk about installments. If you had a job and had payroll tax taken off on each pay cheque, that is like an installment. You are paying the tax at the time you earn the money. Not the next year.

But we don't have payroll tax taken off, we are self employed so we have to pay the 2014 taxes in 2015. Unless... you pay the tax in 2015 on the money you earn in 2015.

So, to the OP, if you owe $5,000 for 2014, you must pay it before April 30, 2015 or else you will have to pay interest. That is separate from paying 2015 tax in installments. The CRA assumes if you owed that much this year, you will owe that much or more next year.