Looking for some ideas on a 5000 sq ft lot

[Calvin Liang]

My cousin owns a property in San Leandro, CA. The lot is about 5000 sq ft and there is a SFH on it (occupies about 1000 sq ft).

He wants to extend the SFH to 5000 sq ft total (about 10 rooms) and turn it into assisted housing. He had dealt with assisted housing before. The rough number he estimates is:
* Construction cost: $350k - $400k

* Total sq ft: 5000

* Monthly Rent: $8k/month

What do you think about these numbers or this idea? We haven't done any construction work, so any input would be appreciated.