Hi.

 

Should the gap between bottom of window to stone sill be sealed?

 

New windows installed and bottom one's are all the same as pic. 

 

Thanks 

Yes and no, yes you need the joint to be weather / wind proof but that is better done further back. My window guy never mastics the underside of cills unless there is a big or uneven gap. They bed the cill into mastic (you can’t do that at the sides or top) so no visible sealant needed.

Some UPVC windows drain along the bottom edge of the window, so if it is that sort of window, then no, it shouldn't be sealed.