Strange "balancing payment"

[Leo123]

Hello!
I just started being self employed last year, so I am applying for my first tax return. I filled all online forms (for 2017/18) and my total income is 14,620. It shows the following values:
Personal allowance £11,500
Total income on which tax is due £3120
Income tax £624
NI £720.69
Total £1344.69

And then it shows:
Balancing payment: £602.52 and then it shows the total I need to pay is £1344.69 + £602.52 = £1947.21.

Why is this and why should I pay these additional £602.52?

(By the way, some online calculator shows I even need to pay even less: total of £1296, which is £650, so I am totally confused).

(I am 18 years old and study in university, not married, don't have children etc.)

[robbob]

Are you sure the balancing payment is a balancing payment and not a payment on account for ye 5/4/2019?
When you tax exceeds £1000 hmrc want payments on account for the following year.
You pay 31/1/2019 and 31/7/2019 - each payment being half the ye 5/4/2018 bill excluding the class 2NI for that year.
Note you can lower payments on account if your 2018/19 tax bill will be lower than 2017/18.

Looking at the figures i am pretty confident 602.52 if your first payment on account for ye 5/4/2019 and there will be another one due 31/7/2019 as things stand.
Note by the due date of payment (31/1/2019) for the first payment on account you will be 10 months into the tax year so its not that unreasonable that hmrc ask for a payment at that stage, albeit its a bit of a pain if you think your tax will be lower but you are not sure how much lower - if that is the case i would recommend paying as close to the original requested amount as you can in january and then sort out the exact figures before 31st July in time for the second schedules payment.

[Leo123]

Many thanks for your response! Yes, my income last year was large just in case and I am sure I will have around 10% of it next year, as I am a student. So if I will not have any income in 2018/2019 they will owe me those 602.52 right? (if I paid that already or I can as you mentioned wait and apply for tax return early, say in May).

[robbob]

You can reduce your payments at any time as appropriate (with the proviso they will be amended backup if tax ends up higher)
so if you are pretty certain you will have nil liability for ye 5/4/2019 you can reduce those payments on account down to nil - if you estimate you will owe £100 you can reduce each payment to £50 etc etc etc

[Leo123]

Thanks!