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The builders "preferred lenders" typically have higher interest rates. Sometimes they mention they will cover your closing costs if you utilize their preferred lenders but many times that means their preferred lenders increase the rate to essentially "cover the costs."
You have quite a bit of time to shop. Lenders interest can be cyclicial. Lenders sometimes reduce their rates below the norm for a few months to draw in more business, then raise above norm. You just have to keep your finger on the pulse and see who's offers are best closer to the time your home is completed.