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Mortgage mandate:
By means of a mortgage mandate, you give the loan institution the power to have a mortgage registration performed without your approval

In rather formal language, the standard expression for this in English would be 'encumbered by mortgage'. Although in popular usage, a mortgage refers the loan you take out to by a property, the mortgage is actually a special kind of pledge that you give to the lendor saying that if you don't keep up your repayments, the lendor can take the property to recover the debt. In mortgage deeds, the lendor may be referred to as the 'hypotheekgever' and the borrower as the 'hypotheeknemer', which sounds strange at first because, according to popular usage, we think of the bank as the party who gives us a mortgage.

In a formal text, you could translate the sentence as 'is the property encumbered by mortgage?' See examples below. In everyday language, you might say 'is there a morgage on the property?'

-> "If any assets of the estate are encumbered by mortgage, pledge, lien or other security interest, the personal representative may pay the encumbrance or any part thereof, renew or extend any obligation secured by the encumbrance or convey or transfer the assets..."
http://www.azleg.state.az.us/ars/14/03814.htm

-> "One tract of land held by the estate had a fair market value of $700,000, but was encumbered by mortgage and accrued interest of $400,000."
http://www.taxlinks.com/rulings/1989/revrul89-4.htm