New construction appraisal

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Hi folks,

I'm currently working on building new construction single family homes on infill and rural lots in my market. 

Several lots I'm interested in do not have "new build" comps, only pre-existing. 

I'm wondering how appraisers usually value new construction when there are only pre-existing homes? Does new construction receive a premium or will they be fairly in-line with pre-existing sales?

Are there any FHA guidelines on this?

Thanks,
Patrick