How do you put a built in oven in place of a freestanding one in a new kitchen?

I am getting a new kitchen put in by my housing association. I would love a built in oven. Do you think that I could pay the workmen money from my own pocket so that they can do it in their "free time", and if not, Is there any way i could (or my bf could!) extend the work surface somehow for the hob, and build a little cabinet for the oven? Or would this need to be done at the same time as the kitchen is being built? Thanks.