Hi guys enjoy what you have to say about this topic.

If the developer can do this and make great profits then that to me says that this a perfect deal/opportunity for you to become a developer and do the 5 house development your self, sure its not as simple as that and takes a lot of planing and organising but if it takes you a year or two to plan and learn everything you need to do the development in that time who says you wont gain another 30k in equity.
You could sell two off the plan to cover costs of construction and go from there, but it all depends on your situation ie family, work and lifestyle, i think you should consider it because you have the property already and why should the developer take all your possible profits that you found first.

again this could be alot more complicated that just getting a 350k house, but if you can do it well you stand to make good money and it could be a life changing experience! you can do it!!!

good luck either way.