I understand that FBD are competetive for this type of situation. You will need Employer Liability, basically covering your liability as employer to any of yoru employees on site. It does not cover injury to employees of employees. Therefore, the carpenter is covered, but any of his employees, should be covered under his policy etc.
you will need Public Liability, to cover non employees on site, ie engineer, surveyor, visitors, peeping toms etc.
you should also insure the building in the course of construction for fire and storm and cover it for the correct sum insured and then cover the building as comprehensively as possible, once it is fit for occupation.