Donald Trump was able to deduct $39.1 million from his 2005 federal income taxes by pledging not to build houses on a New Jersey golf course he owns, according to public records.

An appraisal conducted for the 2005 transaction determined that Trump National Golf Club in Bedminster, N.J. was worth $49.5 million if it could be split into 33 estates, but just $10.4 million as a golf course. Tax law lets property owners impose permanent conservation restrictions and then claim charitable deductions for the reduction in value. Those restrictions curb what future owners may do with a property.

In 2005, according to the Internal Revenue Service, 2,186 taxpayers claimed so-called conservation easements for a total of $1.8 billion. That means Mr. Trump’s $39.1 million would have represented about 2% of all such deductions nationwide that year.

Mr. Trump, now the front-runner for the Republican presidential nomination, subsequently built a second golf course on the property, which once belonged to automobile executive John DeLorean. He said last year in his federal financial disclosure that the property was worth more than $50 million.

The appraisal raises questions about the success of the golf club, in a semi-rural part of New Jersey where Mr. Trump once considered building his own private cemetery.