Your personal level of involvement and knowledge will influence the build cost.
Option 1 - DIY and Subcontractors
Building progresses on a DIY basis and employing technical help where you need it. Materials are purchased directly from builders merchants. This can be very expensive and you won't benefit from trade prices. It can also be the slowest method of completing your build as you will need quite a bit of spare time to manage the subcontractors and get building at the weekends.
Option 2 - Subcontractors only
Building using subcontractors that work directly for you. Again, materials purchased directly from merchants and you will need to manage each subcontractor separately.
Option 3 - Main Contractor
Building using a main contractor. This requires the least involvement from you. It probably costs a bit more but is the least stressful route as all you need to do is liaise with the contractor and he will take full responsibility for making sure the build comes in on time and to budget.