My friend is a building consultant who is experience in A/A jobs and she can help you to cut costs as you need not engage an architect which is more expensive. If you are interested, I can recommend you to her. You may engage someone like her to help draft up design proposals for you. Once confirm and approved by the relevant departments, she can invite builders to tender the work involved. Then you decide who to take up the job.
If not, you will lose out as there will be hidden cost as we are lay man and do not know anything about construction cost.