Brooklyn: Sponsor accepted offer but we decided we want additional construction. Ask them to do it? Or wait?

We have an accepted offer on a new sponsor selling apartment in Brooklyn. It’s a two with a large living room. We want to split the living room into an additional bedroom.

The sponsor is still doing construction on the building. Is it normal to ask for the additional buildout as part of our purchase agreement with the added costs? Is it easier now because there’s only the sponsor as part of the condominium board? Or should we do it ourselves after we are settled in?