It isn't technically, but very few people get prosecuted for it, so they do it anyway.

That was actually one of the fraud charges brought against trump in the NY lawsuit.

Relying on objectively false numbers to calculate property values. For example, Mr. Trump’s own triplex apartment in Trump Tower was valued as being 30,000 square feet when it was 10,996 square feet. As a result, in 2015 the apartment was valued at $327 million in total, or $29,738 per square foot. That price was absurd given the fact that at that point only one apartment in New York City had ever sold for even $100 million, at a price per square foot of less than $10,000. And that sale was in a newly built, ultra-tall tower. In 30 year-old Trump Tower, the record sale as of 2015 was a mere $16.5 million at a price of less than $4,500 per square foot.

I.15.a of this document

https://eddsa.blob.core.usgovcloudapi.net/public/452564_2022_PEOPLE_OF_THE_STATE_OF_v_PEOPLE_OF_THE_STATE_OF_SUMMONS___COMPLAINT_1.pdf