how to finance an infill home on a property I already have mortgaged?

My situation is this:

I am coming up on my first 5 year renewal for a mortgage I have with my bank. The remaining amount is about 50% of the current value of the home/land ($200k owing on a $400k property), with most of the value coming from the land.

The lot is in a great central location, the home however is a run down post-war 1.5 storey. We've been exploring renovation options and I've secured a Heloc from my bank to this end, however the pricing we're getting for renovations makes me think a new build would make more sense. I estimate the new build would cost between $160k - $200k at a cost of $125 - $150/sq ft.

What are my options for financing the construction of a new house on a property I already have a mortgage on? The new home would ideally use existing foundation (which is in good condition) and be about the same square footage but with a reconfigured floor plan and modern materials and finishing. I would like to tie in the financing of the new build with my renewal that is coming up in June.