Can you change planning on site with planning permission ?

Site with planning permission

Hi all just wondering if anyone has any advice, currently looking at buying a site not in a rural location so no local needs etc criteria. It has planning permission already however if we're to purchase the site we would want to change the plans build a bigger house etc is this possible to do? Or would we be making things to difficult for ourselves even trying as we can't afford to end up with a site and not be able to build what we would like to on it. Appreciate any advice from anyone who may know! Thanks