On the development, the big difference is that you pay corporation tax on the profits rather than (probably, but not necessarily) income tax. However, you have to pay more tax (either dividend rate income tax or CGT) to extract money from the company so there's only a benefit if you expect to keep the funds in the company long term. Bear in mind CT could rise from its current low by the time you build and sell the property.
It's possible (I think) you could get CGT treatment on a personal build and sale but not if you bought the land with a view to selling it on. CGT rates are also at a historical low and I reckon they are a good candidate to go up from 28% next year.
When it comes to the existing BTL, I'd suggest you get an accountant to run the numbers for you because it all depends on the circumstances - particularly if you have mortgages on them.
Thank you for the info... Whenever I have looked at whether to incorporate, I could only see additional taxes rather than savings.. though it makes sense that if keeping it in the company, the taxation does not apply enabling you to grow the company and only paying (higher) taxes when drawing down. Though with the mortgage interest treatment, there is an advantage there I'm sure.
I will need to think carefully about whether I am keeping the funds in the company... I might do medium term (5 Years) and use them for additional developments / refurbishments...
I hadn't thought about which taxes will rise in the years to come, but I reckon you are right about both CT and CGT.... which may sway my end game plans... choosing to keep it and rent it out... perhaps.
ps. delay in responding as I didn't receive any notification of a response and I've been rather busy. I appreciate all advice I get here so much.