It had a HUGE lot, the building was in excellent condition!

What condition was it in?

If the condition of the property warrants it, raise the rent to market rates once the existing leases are up.

DIY you can write out all the terms and conditions on a yellow pad and both sign it.

The cost is going to be less than 30k because I will build it myself but obviously the county won't just accept my "random" estimate, they will allow to estimate $123/SF of conditioned space and $22/SF of deck.The floor plan I was planning on building is coming at $37,404 (288 SF floor plan, 90 SF front deck), so my question is : shall I make it smaller to get below that 30k limit ?

My weighted average investment property loan is right around 5% (from 4.75% to 5.25% on 30-year fixed).  

Minimum standards include, but are not necessarily limited to, the following items:The unit must be kept in condition such that one person is able to clean within four hours.

If it is the former, pull comps and look at the condition of the home to back up your lower offer. 

At that time it was a buyer's market and it was impossible to sell the place FSBO at a non-short-sale price and realtors were actually turning me away because my place didn't have the curb appeal that they thought they would need to attract the few buyers there were in the market (opposite conditions to today where every realtor is thirsty after a seller lead!) 

You can filter by the beds and baths and other aspects to make the results as close as possible to the one you're comparing.When I build my semi-automated hotsheet for investors, I take exports from the MLS from both the sales side and the rentals side, pull them into a SQL Server database, do some data cleaning to screen out as much of the garbage as possible, and run what I call my "magic query" that joins sale listing locations to rental listing locations by a conditional match on subdivision and beds/baths, or the average of the entire MLS if the subdivision is given as "METES & BOUNDS", with a flag in the output showing that those M 'n B rental numbers are MLS-wide averages for a given beds/baths and will need a more in-depth evaluation (like rentometer) if the estimated cash on cash return for a classic 20% down scenario, looks promising.