Madrid-based company RR de Acuna & Asociados told Bloomberg there are about two million unsold homes in the country, due to the boom that saw about 675,000 new properties built every year between 1997 and 2006.
"Stock isn't being reduced and some of it will never be sold because of its quality and location," Fernando Rodriguez de Acuna Martinez, a partner at the company, told the news agency.
He added that some homes "may have to be demolished in the future to stop the slide in prices".
Recent drops in Spanish house prices could provide opportunities for investors, with the National Statistics Institute in Madrid reporting a 14.4 per cent year-on-year fall during the second quarter of the year.
The institute's latest house price index also showed that average values declined by 3.3 per cent from the previous quarter.