One of the simplest and most practical ways to help solve Denver’s shortage of affordable housing may be found down the hall in a typical home, in that unused room with the door closed. Spare bedrooms typically rent for as little as half the cost of a one-bedroom apartment. Getting some of that extra supply into the market could bring down rents and ease the pressure on home prices, while also boosting the income of homeowners. But accomplishing that goal could take a shift in thinking among homeowners — particularly older ones who have extra space and could use the rental income — and efforts so far have had limited success. Of the 723,000 homes that are owner-occupied in the metro area, about 442,000 have spare bedrooms, according to an estimate from John Burns Real Estate Consulting. And of those, up to 142,000 are owned by retirees, who typically live in households of one or two and who might benefit from rent income. Real estate website Trulia is more conservative, estimating that older homeowners in the metro area have about 72,000 available spare rooms. “The biggest supply of vacant housing is empty bedrooms in existing homes.