Some thoughts:
1) When is your close of escrow? If it's greater than 60 days, the quotes you will get are great.... for about 30 minutes... then they vanish as rate change hourly sometimes. Wait until the 61st day before closing to start shopping.
2) Consider when your last Mortgage Company Sourced Credit Report was run. If you're in a long escrow, the builders lender ran your credit 4-6 months ago. If your score data is more than 90 days old, see if you can get updated scores. Do not rely on the score data that Credit Karma, Discover, or other "Free Score" sources provide. They can often be 20-30 points off from what the mortgage lender has. Do you need each lender to run your credit to get an accurate rate? Nope. If a lender refuses to assist without a credit report being run (see 4 and 5 below) you have your answer as to if you want to work with that lender.
An article of note on "Free Score" accuracy:
http://www.investopedia.com/articles/personal-finance/103015/are-credit-karma-scores-real-and-accurate.asp
3) You've got a rate quote. Do you know if you're mandated to have an impound account? More than likely the answer is "Yes". If you do not want an impound account, ask whomever you speak with to structure the quote without impounds.
4) When comparing, an e-mail saying "today, our rate is X at zero points" is not a deliverable or comparable rate quote. E-mail pricing isn't enough data to know precisely what you will end up with. That said, if you can only get rates by e-mail and not in a "Cost Worksheet" or some other form of estimate, avoid working with that funding source. They are hiding something, or won't take the time to assist in a professional manner. Pricing may look good, but if they aren't going to put it in writing, that's not who you want funding your deal.
5) Get Cost Worksheets from the Builder's Lender, your Bank, and one other resource all within a 24 hour period of each other. That's as close to accurate, comparable, and deliverable rate data you can get. Without everyone being on the same and level playing field, I can assure you one of those lenders is lying about their pricing.
6) Ask about other ways to reduce your rate. Some lenders will lower rates with an automatic withdrawal, or diverting your payroll deposits to that bank. Some will reduce rates if you move $$$ to their institution. Ask not... get not... so be sure to ask.
7) If rates improve, can you "float down"? The best answer is "No". That said, lenders will want to retain your business if rates move in your favor. The general rule of rate lock breaking is this - should you lock at 3.50 percent, and rates drop to 3.25 percent, the lender could - in some cases - relock your rate at 3.375%, splitting the difference with you. This price improvement is a one time deal. Also, when a customer hears "Rates are lower" - what is the context for that? A lender might have 3.50% for zero points, and a week later 3.50% is at a .25 lender credit. That's a lower price, but not a lower rate. If you hear "rates are lower" be sure to source this information, or rely on your lender to have this data at hand. I use Mortgage News Daily, and this chart:
http://www.mortgagenewsdaily.com/mortgage_rates/daily.aspx
Note how little rates have moved in over a month! Granted, this is a big data set, and there can be one or two day punches up or down, but all in all the changes have been very slight.
Follow these few guidelines and the most competitive and competent lender will clearly emerge. Note that I didn't say "lowest rate lender". What good is the lowest rate if it's provided by a company that won't do what's listed above? What good is the lowest rate if they primarily process refinances? What good is the lowest rate if they don't know new construction? Is the Lender competitive? Yes. Is the lender competent? Yes as well. These two criteria tend to be the more important guiderails to bear in mind when starting to shop for a mortgage and committing to a final service provider for one of the most important financial decisions you will make in a lifetime.
My .02c
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