You can verify if a permit was pulled before the work was started in most towns / cities by calling the building department of the town you live in. 

We contacted the city building department, and were told it needs both zoning change and permit.

Dear Mr.

After 40 years of marriage, the only words you need to know are "yes dear".Initially, I signed on with KW in South Portland, Maine but quickly found myself literally ghosted when I desperately needed support - including buying my own home! 

Call your zoning department and tell them explicitly what your concerns are and see what they say. 

Start by going down to your county clerk's (or city's) land records department, they can help you look up the property information. 

The department of HUD also has a website listing its analysis of market rents on every city and town in the U.S.

For example with the building department having cited it for non-compliance.   

That may work, but it may also come back to bite you and cost you dearly.

Others have touched on the flip side of this above (I agree not a great flip either), but here are the numbers from my Philly market analyzer using a hard money/private money lender and then refinancing out for 75% afterwards with 6 months of construction (which is probably not long enough for that job given how slow Philly License and Inspection department is taking with Covid).