With a rehab loan, and when you own the home already, you usually start out by selecting your contractor to bid out what exactly the improvements are going to cost, what types of materials would be used, etc. I usually recommend you get at least 3 estimates but for a huge job I'm not sure if estimates would cost money to get since they'd be quite involved. But that would be step 1. After that is done, I'd recommend you contact an appraiser or real estate agent who really knows your neighborhood, as you will be looking for some sort of estimate of your home's value after those items are completed. This may involve taking someone out to lunch in trade for their time/expertise. You can skip this and just wait until the appriasal is done, but me personally I like to know as many facts as possible before delving into the loan application. Anyway, that value figure will be important because it will determine what the maximum loan amount you could potentially get to do the improvements (and pay off your existing mortgage). Next you'd want to apply for the loan. You could go to someone who only does the 203k, or who only does the Fannie Mae HomeStyle, but I'd recommend you work with a loan officer who does both (and does both well). When you approach that loan officer you will have pretty much everything they need to start crunching some numbers off the bat - the current home's value, the cost of improvements, the type of improvements, and the home's estimated value after the items are complete. Then the rest of the process is qualifying just like any other mortgage, you got your credit, income, debts, etc. There is a part that involves getting your contractors approved, including reviewing their license, resume, their general liability insurance policy, etc.
has some great reading on the FHA 203k loan.
has all of FHA's resources on 203k loans.
Fannie Mae HomeStyle renovation will allow renovations/repairs totaling up to 50 percent of the as-completed value. They have a nifty form you can download that will help you calculate the maximum new loan amount. https://www.efanniemae.com/sf/formsdocs/forms/1035.jsp
if you are aware of the program guidelines.
(page 4) are those guidelines.
is a list of "Consumer Tips" Fannie Mae has about their HomeStyle program. There is also some HomeStyle Resources (including a recorded presentation on the loan program, put together by Fannie Mae) at https://www.efanniemae.com/sf/mortgageproducts/pdf/hsrenofac
Specific details about it can be found at https://www.efanniemae.com/sf/guides/ssg/sg/pdf/sel012711.pdf
(VERRRRY large .pdf - page 682)
I have a done a few 203k loans, and I have done 1 HomeStyle renovation loan, so I wouldn't say I am expert at navigating the process... but it isn't terribly difficult as long as the loan officer knows the guidelines (as well as the lenders "overlay" guidelines). Plus I don't offer the renovation version anyway, so I don't believe I'd be the "man" for the job. However when the time comes, if you do need a recommendation, I can find the type of loan officer you would need.
Hope this information has helped.