Self assessment tax query

[SidewaysSi]

I have a couple of buy to let properties and filling in the self assessment, I have made roughly £12k profit (includes about £6k expenses in mortgage interest).

The online calculator is saying I owe £8.5k for the 2016-17 year and another £4k for the first 6 months of 2018. I am a 40% tax payer through PAYE.

Doesn't sound right to me? Am i missing anything obvious? Think I need to find someone to verify my return...