Plumbing work progresses at the Concord, Mass., barn conversion (including the last-minute addition of a first-floor shower), but a proposed stairway causes design problems. Meanwhile, host Kevin O'Connor tours recently renovated guest cottages in Nantucket, Mass. On “Ask This Old House,” projects include a new ground cover (ivy and pachysandra) for a forlorn lawn; and opening double-hung windows that have been painted and have swollen shut.