It is shitty, but so far as I can tell, accurate: https://www.helgilibrary.com/indicators/mortgage-loans-as-of-gdp/usa/

If mortgages are lumped in with credit card and automotive debt, so-called "household debt", the figure jumps to 73% debt : GDP per the FED. https://fred.stlouisfed.org/series/HDTGPDUSQ163N