Learn French: Je préfère le sous-sol, pour lequel nous avons un beau projet. - I prefer the basement, for which we have a beautiful project.

In French, pour que is a fixed phrase meaning "so that" and it forces the next verb into a different mood. When you want to translate "for which" referring back to a physical thing like le sous-sol, you must use the preposition pour combined with the relative pronoun lequel.

Why is it exactly lequel and not another form like laquelle or lesquels?

The relative pronoun lequel must always agree in gender and number with the noun it replaces. Here, it stands in for le sous-sol (the basement), which is masculine and singular. If we were talking about la maison (the house, feminine), we would use pour laquelle.

In English I could say "which we have a beautiful project for." Can I put pour at the end of the sentence in French?

No, French never allows you to leave a preposition at the end of a sentence or clause. The preposition (like pour) must always immediately precede the relative pronoun (lequel). It is strictly pour lequel (for which), never lequel... pour (which... for).

Why do we use beau here instead of bel?