I've done dozens of renovations now and I still use the 20% buffer and because of that I'm typically able to come in at or below my budget.  

Likely if you can gross enough and have a buffer you should come out ahead in the end. 

Always add a 20% buffer when it comes to construction.

We have felt that we need that much of a margin, more for a buffer against risk than anything else.

I have no problem leaving 20% of my personal finances into the game and almost prefer to for an appropriate buffer with equity etc.

(especially when you don't ask for good reviews there is no buffer)  @Jim S. 

you need to make money when you buy.so all good on apts but no need to rush unless you get a value add opportunity .also make sure you have a buffer of 50 to 100k to deal with the surprises.

I've put in buffers for management company, vacancies, utilities, etc.Has anyone else ever purchased a college rental of this size?

Now you have some time buffer but still need to ensure that you can make the payouts for the second year. 

So, if it costs $200K to build a home, you should set aside $6K-$10K as a buffer.