I don't know the legal structure of Ethiopia. But all in all company registration is necessary or aa necessity in this case so that you can be able to bid big and minor contracts for your company. That's how it can grow.
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The hardest part in my experience is finding good workers and getting paid fast by the customers.
Good contractors are always in demand since too many are not reliable and don't deliver the work on time. So, the better your staff and subcontractors the better you are. When it comes to getting paid, try to get as big a deposit as you can before you start.
Also do not too much work without getting paid in case the owner stops paying or paying you slowly. Your workers want to get paid every week and if you don't pay them they might leave. Your suppliers want to get paid and if not they will take away any credit they might have extended to you.
Another way to grow is get credit from your suppliers then you don't have to pay cash all the time and can wait a little for your client to pay you.