In a statement, the Yorkshire Dales National Park Authority (YDNPA) said that the letter stressed that its planning policies "have recently changed, with one of the main objectives being to increase housing supply".
The Yorkshire Dales National Park Authority adopted its new local plan in December last year.
The authority said that this "new, more flexible local plan sets a target of getting 55 new homes built in the National Park each year".
It said that, since the plan’s adoption, plans have been approved for 79 new homes, "with new policies on converting roadside barns and other buildings to local occupancy dwellings accounting for 50 of these permissions".
It said that last year, 39 homes were built in the park area.
YDNPA chairman, Carl Lis said the authority wanted to "nudge builders into action".
"New build housing is now permitted in more towns and villages across the National Park than ever before.
"We have put together a long list of sites that have the benefit of an existing planning permission or housing allocation. Our message is simple: if you bring forward high quality schemes for these sites, we will approve them."
Lis added that other policies now "permit more open market housing than before, making it more financially viable for developers to build the affordable and local occupancy homes that are so badly needed by local communities".