If you had 80k in household income and no debt other than existing mortgage, I would guess they would give a mortgage of 300 - 350K total.
When you say 2nd house, I assume you mean second investment property with expected rental income. You already have a mortgage of 280K, so you do not have a lot left out of 300 - 350K. My guess is that you would need to put forward a very convincing case for the rental income you would get from the 2nd house to have a chance. The banks view would be that if you don't secure strong / secure rental income, you would be left in a pickle. Banks are more interested in ability to pay than the equity you bring to the table.
I'm in a similar situation with the same approx household income, similar equity, but my existing mortgage is only 50K (house value 220K approx). Buying a 2nd house for the same value as my home only means a mortgage of 270K (excluding stamp duty, etc). I don't have to be so convincing on rental income, if at all.