Do an assessment of value now and after you get the ADU permitted * We have been in situations where $8k for permits could mean an extra $100k in value or $2k per month in rents. 

Quote from @Trent Reeve: Quote from @Bruce Woodruff: Quote from @Issa Joseph: I DIDNT GET IT LOL for now my business plan doesn't support this purchase but an extra 10k per unit per year profit is significant growth in revenue But ask yourself - how much of this $10k increase (remember alleged) could you gain yourself by doing research and asking questions on this forum.......? 

If the plumber has to work 2 extra hours because of foolish clogs, oh well. 

Other asset classes aren't so the depreciation schedule may not be as quick/beneficial.2) And it is even possible to find deals that are what I call "super-shielders" and which not only shield all the distributions in the investment but have extra to use over on other real estate investments that are throwing off income. 3) Unless you're doing a 1031 exchange  at the end or similar tax structure to defer taxes, you will have to *pay back* all that depreciation benefit at the end (on sale...which for you is in 5 years)  via depreciation recapture.

Typically how  much extra should I budget for, for these unknowns? 

For me, this is just a few extra bucks I have to pay to the CPA industrial complex.

I will also note that the RC zoning district allows for quite a few extra units so you would get the best use out of your land by building a quadplex or something like that.if you are interested in going forward though I work part time at NOLA builds in town and they do great work for construction at all levels.feel free to reach out if you have more questions 

I would only do it if you need the extra $3k cash flow this year less the cost of doing the segregation.  

There is extra costs, more regulation, and more liability. 

The important part about cash flow is that you stay solvent; bring in enough to pay your bills and a little extra for a rainy day.