So there's two elements I would suggest to focus on here:CostsResponsivenessCosts you can compare by analyzing the "lender fees" on a "loan summary" or "loan estimate". 

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 ?

Just a few thoughts: - Get a thorough inspection- Get 3-5 estimates from GCs- Pick the right ‘genre’ of GC (fourplex and smaller usually falls under midsize or smaller GCs) - Always check license numbers and if you’re repairs or more than $25k, request references/info for 3 past similar projects with pictures or check social media and reviews - Get a service agreement in place before finalizing your deal and make sure the estimates cover everything in your inspection report and nothing is overlooked so there’s not as much unexpected cost that pops up.

Check out the book, the Flipping Blueprint it can give you some guidance on flipping estimates, it depends on the level of rehab there are some rough numbers once can use as a price per sqft.

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. 

@Lee Ripma  The percentages can actually be higher than the estimates above with a quality, engineering-based study, the IRS' preferred methodology.

That is great that you could do all that work for 45k, helps me to get a good estimate on how much these sorts of jobs can incur.

You also should get a GC in there as you should pull permits for the plumbing if it’s all been pulled out so you can have an estimate on the full scope of work.

So roughly a month and a half to two months I would say is a good estimate. 

Zillow provides a decently accurate rent estimate too.