Clarification on buying flat and parking space separately

Hello to all. my builder made an agreement and in which he mentioned that flat with common rights and car parking space. so, in this case he assured me that i can get my flat and parking space too. my question - it"s legal? and if i buy parking space seperately then i can save some registration fee? someone told me tHAT if i purchase parking space with flat then the registration fee will be charge on the flat price only and my flat and parking space price are different. so, what will be correct - should i buy flat with commons righ and parking space or flat and parking space differently - which one is better and cost effective?