Is it legal to rent an office and use as home to live in ?

I want to have an office but I can't afford an appartment and an office, so why not rent an office and use one small room as a bedroom ?

In general the answer will often be no. It depends on what the property is zoned for by the governing authority (usually a city government since you're talking about an office).

However, it's certainly possible that buildings be zoned both commercial and residential. But you're going to have to be able to get a Certificate of Occupancy, which will require for instance a bathroom, probably a stove to cook on, etc. A situation like that will typically have the residence on the second floor, and commercial space on the first floor. These used to be more common than they are now.

Ask your city building department, and they will provide the answer.

because the code does not allow it. Your county's zoning board sets forth what can be done and not done.

I would get an apartment with an extra room and make the room an office. Once you are making money with the business - then rent an ofice space

Most offices don't offer shower facilities. So that's one problem I foresee, right off the bat. I would also check with the lease to see if that's allowed, you don't want to find out the hard way...

Most office leases won't allow it.

Usually not. Need to check zoning ordinances, and rental/lease agreement.

LOL-rent a larger apartment, convert the bedroom into an office. It would be foolish to live on commercial office property.