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It is generally about 30 cm, and there are beams hanging flat with beams. Thank you.
No more than 20 cm.
It can be done, the width is 30 to 40 cm, you don't need to install the light strip, you can hang it down 10 cm, and then walk around the plaster line.
With a clear height of 3 meters, you can choose any plan until you like it - the current high-rise is also meters.
Keep the edge of the circle and remove all of the middle. The height of the floor will be visually elevated a lot. You must be determined!!
It is a perimeter ceiling, not much gypsum board, but it will be more time-consuming and laborious, wood and light steel keel, I recommend or light steel keel. The price is not easy to say, it is not a very large area, and there will not be a lot of materials and labor.