Homes that are unusual because of design or building materials are a challenge to finance, bankers says.

Lenders are paying stricter attention to home values than they did several years ago. It's simply a fact of the mortgage lending environment in general that determining value on a property is more important than ever today, said Brad Blackwell, a national sales manager for Wells Fargo.

Environmentally friendly houses, custom-designed by their owners, are getting caught in the valuation retrenchment because of the lack of comparables. A house in Colorado made out of discarded tires compressed into building blocks is a good example. With no other houses quite like it in the state, assigning a value is a challenge.

And new rules from Fannie Mae and Freddie Mac, which are intended to reinforce the arm's-length distance between those assigning an appraisal and those who conduct them, complicate relationships some mortgage brokers have forged with appraisers who have familiarity with valuations for odd houses.