"according to building code, we cannot have a structure that is to be constructed equally to the lot area"

is it true?
Ozcity:

Yes it is true. You can only construct a building not more than 80% of the lot area. The remaining 20% is reserved for the open spaces(setbacks) of the lot. Moreover, the building construction depends on the type of lot eg. Interior, thru, etc.

So the rule of the thumb is do not construct more than 80% of the lot area. However, You can contruct more than 80% of the lot if you build a multi-storey building. But the rule still stays in each and every floor.

eg. 100 sq.m. lot

You can build 150 sq.m. of building area (80 sqm. ground floor and 70 sq.m. of 2nd floor).

Also, consider having firewalls if you allow the maximum buildable area in your lot and check for building restrictions in your area.

Hope this helps.