Everything starts with the plan, finalize everything with the plan from the sizes of the rooms, the locations of the kitchen, living , dining rooms etc. You cannot have an accurate estimate without the plan, from the architectural, structural, electrical, sanitary plans, that must be complete to have an accurate estimate of the cost. As an architect by profession, as long as it is on paper it can be changed with minimal cost, but if you start building and changing the plans it is going to cost you, more if you are nearing completion. The cost per squaremeter is only a guide, as long as every thing is in black and white, it can be computed down to the last bag of cement, to the last nail in construction. My advise is to plan everything first before you start, to get a ballpark figure you must finalize the plan.