A 2-family is legal if[7] Two-family household living is permitted in the CMX-3 district provided at least 50% of the ground floor frontage contains a non-residential use, or the lot area is less than 1,440 sq. ft.https://www.phila.gov/media/20...

Average household income greater than 3 times average property monthly rent annualized 5.

I typically looks at median household income; both recent data and % change from last census

on those sites, check average home price , average apartment rents , average vacancy rates , unemployment , household income and then I also like to Zillow or trulia the current properties for sale , for rent and that gives you a good preliminary on the market I think. 

See Garn St Germain"A lender may not exercise its option pursuant to a due-on-sale clause upon--, (1) the creation of a lien or other encumbrance subordinate to the lender's security instrument which does not relate to a transfer of rights of occupancy in the property; (2) the creation of a purchase money security interest for household appliances; (3) a transfer by devise, descent, or operation of law on the death of a joint tenant or tenant by the entirety; (4) the granting of a leasehold interest of three years or less not containing an option to purchase; (5) a transfer to a relative resulting from the death of a borrower; (6) a transfer where the spouse or children of the borrower become an owner of the property; (7) a transfer resulting from a decree of a dissolution of marriage, legal separation agreement, or from an incidental property settlement agreement, by which the spouse of the borrower becomes an owner of the property; (8) a transfer into an inter vivos trust in which the borrower is and remains a beneficiary and which does not relate to a transfer of rights of occupancy in the property; or (9) any other transfer or disposition described in regulations prescribed by the Federal Home Loan Bank Board.

I realize an increase in rent and a one-time $200 fee might tax your household’s budget, so I am willing to work with you on a payment plan if you need additional time to come up with the pet fee.Please fill out all the highlighted parts of both documents.

Below are the Networth and Household income % in the United States.

Also look at the household income for that area, compare with the rent estimate for your property, to make sure you don't have the most expensive rental on the block or the cheapest either.

Everyone living in the household that is over 18 must fill out a rental application and they each must pay for a background check. 

Interesting (and relatively short) read in the current issue of The Atlantic regarding the increasing population of renters (vs. homeowners), rising rents and the potential impact those have on household finances.http://www.theatlantic.com/business/archive/2015/09/a-bleak-future-for-renters/406453/