* If you're looking at a planning gain. You will need 10 - 12 weeks from the point of completion before you start work, factor that into your holding costs.
* Don't forget the contingency, so many people think they'll add a small percentage of around 5% but believe me this closer to 12%
* Ask your builder to sign a contract, where you clearly have staged payments for jobs completed. PLEASE vet your builder, if you have someone who is much cheaper but has no insurance, registered address etc. this can cause problems (do your homework). Make sure your builder is aware of the required building regs as this is important. Add penalties if jobs run over (on a good faith basis though as building is weather dependent).
* If you're doing multiple properties, get a refurb blueprint. I would stick to the same kitchens, paint, flooring, carpeting etc. as it makes it so much easier from a maintenance perspective to source replacements at a later stage.