If that's not the standard in your area, the potential for the extra rent is attractive.

The extra cabinets I needed in order to complete the design ended up costing $2,000.

I have a W2, so the extra rental income would be reinvetsed into the property.

Cities usually don't care if the OWNER lives in the basement, just tenants.You could use the extra income to offset cost of finishing basement and more.

I had another person try to bargain me down to $1700/month, and said she was moving out of her dad's house and so needed room to store boxes in the extra bedrooms. 

What I will do is attempt to find a QUALIFIED substitute tenant, credit the original tenant with the rent paid by the new tenant LESS one or two months fee for the extra work and effort, but I will NOT release the original tenant from liability should the substitute tenant default.Look, the bottom line is that renting to tenants is a business, just like any other business.  

One was making an extra $600/month while covering all of his household expenses with just 3 PadMates. 

Preapproved ADU plans offer extra space or rental potential.

After listening to him talk for close to 45 minutes, I finally said that if I have an extra $18,000 to $35,000 wouldn't I just invest that in my own property and get all of the rental income and if I sold the property 100% of the proceeds, not 50% of it. 

@Aus Smith if there is base rent and then an extra fee the resident pays for a common utility, we count that as revenue towards a percentage-based management fee.