When it came to housing, Susan Apel and Keith Irwin thought they had planned adroitly for later life. They bought a four-bedroom house on 2 acres in Lebanon, New Hampshire, 24 years ago, and “we made sure to pay off the mortgage before we retired,” said Apel, 71. That way, the home equity they had built up — they estimate their house is now worth about $700,000 — would allow them to sell and downsize into smaller, more manageable quarters when they needed them. That time has arrived. Apel, a