If the new build is for your own occupation and is under 125 sq.m. then it will be exempt from Stamp Duty. What you do with your previous house is irrelevant.
I would add a caveat to Liam's statement. If you purchased your old house as a First Time Buyer and now rent your old house out, you could be liable for a stamp duty clawback.
I would add a caveat to Liam's statement. If you purchased your old house as a First Time Buyer and now rent your old house out, you could be liable for a stamp duty clawback.