On the point of staying on budget, I would stress that clear, itemized and detailed scopes of work that include who is responsible for permits and what happens during delays are very good ideas.

There's lot of things that require government approval.I have a dozen new duplexes going up in my area, the area is gentrifying, and at each stage, they post notices that permits are pulled, the permit #'s.

I have a line on my first BRRRR purchase in Huntsville and have a question regarding permits for obvious additions. 

If zoning permits, this will give you the permission to allow it to be a legal unit.   

Depending on age of the home once you pull permits and start taking down walls or open things up you will have to bring things to code which can blow your rehab cost.

This is also a "single family" home (according to permits, according to zoning, and according to MLS... but tell me if anything jumps out at you when you look at the front door), also in Oakland, also no kids:I have $20 that says if you went to both houses and looked in main floors, the basements (first one has epic basement), and attics (2nd one has epic attic), you'd find folks living there. 

In my backyard is a single car garage that was built in 1959 via permit after the construction of the house in 1924.

Also, at least in Florida we have to obtain a special permit for an RV Park/Mobile Home Community.

@Eric Ippolito  first it takes Capital to fund the due diligence process and bring the project to shovel ready but the following steps are free and are done by you to determine feasibility.Check with the city or county to determine what is required for all approvals including site plan and building permits, proffers, water/sewer tap fees, bonding requirements, inspections and CO process and time frame for all approvals.You also need to check with the utility companies and get an idea of availability and cost estimates from them for water, sewer, power, gas, cable, installation and connection requirements, tap fees, hookup charges, transformer location and relocation, power line and power pole relocation issues.Check to see if you have to install any manholes,fire hydrants, curb, gutter, sidewalks, street signs, street lighting any specific street design or access requirements,.Check DOT requirements for access, stop lights and permits, traffic studiesDWQ requirements for permits, permit fees, time framesThis is a broad overview of the process and your civil engineer can handle all of this but it's good for you to know exactly what's required.

The fourth unit is probably not permitted, hence why the owner has to vacate the unit for the inspections.