What should one consider when looking to install radiant floor heat?

We're looking to build a new home and we're contemplating having radiant floor heat installed throughout the property. Right now, the plan is for ~1500 sq ft, one story, probably with a crawl space in Northern California (not extreme temperatures).

What are the key things we should be taking into consideration as we design/cost the project?

What is the "best" source for the heat? Presumably, tankless water heaters (on-demand) are cheaper for overall hot water. Are they suited (if sized correctly) for this type of application too?

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