Nice plan. As I am also no architect, I hesitate to give advice on the layout or design, but it does seem that you are making very much of the space you have at your disposal. (Even if I think you should let your sister have some furniture and windows too [:)] ).
But to get a better impression of the final result, maybe you could:
- add some texture to the outer wall,
- put on a roof (on a new level so it does not interfere),
- indicate possible neighbouring houses with boxes (that you can group and hide with the visible checkbox),
- set compass (it seems to be set to northern France), Yaoundé should be N 3,9 E 11,5 something and timezone GMT +1 (using the European time-zone CET or Paris, might give you a daylight-saving offset in the summer),
- with correct compass will make photo rendering can give you an idea of light and shade (you can also do a full sunlight simulation).
As to whether the project is possible, that is pretty hard to tell without knowing local regulations and standards. But making a similar building (maybe with slightly thicker outer walls and floors) certainly looks doable. But, a good builder/architect will be a better judge of possible snags: To take one example, meeting the requirements to a staircase is often a challenge: the spiral staircase in your plan is narrow / steep to serve as main access to the first floor. Fire escape, fall hazard, disability access - lots of considerations. The diameter would need to be roughly 40 cm wider to have an ascent line in the middle of the staircase of < 42°, the balusters are too far a part to prevent a child to fall through / get head stuck).
But don't let details put you off. The concept is great. And please keep posting progress. Fun to see. Best of luck.
ok