Stamp Duty ... Another Story ...

To all SD experts,
I'm renting out my house now which I lived in for 6 years as a FTB ...
I'm building my new PPR which I have calculated the floor area to be just under 135 sq/m but I'm not 100% sure how it is calculated ...
Can someone tell me how floor area is calculated ??
Do I just add up the living floor area in each room ?
Should I include the area of the internal walls ? (They all add up!)
If I'm not correct with the 135 figure I might be closer to the <125 sq/m
stamp duty exemption floor area ...
If I am correct what ST should I be paying ...
Thanks in advance guys ...

You don't pay stamp duty on a self build house, no matter what size. If you own the site, you probably paid stamp duty on the purchase unless it was a sub divide of land already owned by you.

mf

Thanks for that mf1 ... Honestly didn't know it was that straight forward ...
Anyone any ideas on how the floor area is calculated ??