you should have Employer Liability cover to cover any of the workmen you have on site as you are the Employer.
You should have Public Liability cover to cover non empllyees on site, such as postman, deliverymen, anyone you are bringing on to give quotations for work and believe it or not, any member of the public having a look at the house in course of construction.
You should also have fire and storm cover on the biulding as it is being built. that will protect you in the event of storm or fire damage on the house in course of construction.
you might also like to insure the tools being used/hired in, if your property or being hired in your name.
Please note that any motor vehicles on site, such as JCB, dumper etc should also be insured by the owner under a Motor policy.