Is it posible to know the complexity of a website before creating it

Answer 1

Yes definitely. Before creating a website or starting a project you should always have a good understanding of what it is you want your website to do and you should also have a good understanding on how you should do it. If you jump into a project with no planning then you'll likely end up confused and lost.

Answer 2

If you are creating a well planned website then it is possible otherwise just like Ybot 765 said you'll most likely end up confused and lost.