By 125 years, he might be referring to a law that (they say) allows them to violate the DC building height limit, or maybe this other law (just 101 years old) authorizing another unbuilt project they say means they don't need additional approval for this one.
www.washingtonpost.com/politics/202...
washingtonpost.com
Trump officials say they can build 250-foot arch without Congress’s approval
The administration say they are building on a project authorized by Congress a century ago. Lawyers and Democrats said the argument is an attempt to evade federal law.
Interior Secretary Doug Burgum says they're starting excavation work on Trump's arch