What you need to do is click on the lot you wanna use, and click 'Build Mode'. Then on the top left hand corner, there will be a bit which says 'i' and click on that. Then click on the lot type (where it says 'Residential') and then you can change it. The game will give you a checklist of items which you need to fully make it, I've done it before and made a pool myself, which attracted many sims!
Happy simming!
