Smaller than Planning & calculating sqm for costs

Hi there


Working on a simple build design. Have not submitted for planning yet, but wondered if anyone can help with these queries.


1. If you have planning granted, can you then decide to build smaller without updating planning? (IE submit planning for the ideal want, but if that is too €€€ scale back dimensions while sticking to same design just smaller rooms etc )



2. To calculate sqm of a house in terms of building costs, is it based on the Gross internal area or on the full external footprint?

( For example External 11m x 6 m = 132sqm VERSUS Internal 10m x 5m =100sqm) that is 30% of a difference which would impact massively.



I know the whole 'price per sqm' is ridiculously vague anyway as it depends on the whole picture, but I am trying to configure what we want with our budget in some (very loose) terms before we get into the official drawings stage.



Thank you