I just seen one recently constructed in a terrace house. It is a simple loft about 10 X 10 feet.
The side is supported by steel beam and is anchored to the huge roof concrete beam at the top.
Floor is made of plank.
A ladder is used to access. Likely used for storage.
BCA approval is needed.
If the loft is huge, foundation beam to the floor might be required.
I have been thinking of building one too.
Along upper changi road, near to Kembangan MRT, you find many hardware toiletry stores.
Shop around, prices are reasonable and they can recommended workman to install as well.