Salarpuria Sattva Chansandra: Hidden costs beyond booking?

Budget finalised, loan pre-approved — ab अटक गया hoon on hidden costs. I'm looking at a Salarpuria Sattva project in Chansandra. Their 'all-inclusive' price sounds good, but log bolte hain there are so many extra expenses beyond the property price. Like stamp duty, registration, interiors, society charges... I had no idea kitna add ho jayega. What's a realistic percentage to keep aside for these? Also, any site visit red flags builders won't volunteer? My parents keep saying check plumbing, but aur kya important hai?