Hundreds of families living in housing subsidized by Fairfax County taxpayers exceed income caps designed to ensure that only the neediest receive assistance, a review of county records shows. In the most extreme cases, Fairfax is underwriting rents for families making well into six figures: One household getting help makes more than $216,000 a year; another, $184,000. Dozens of others -- making $60,000, $70,000, $90,000 -- exceed eligibility caps. And they do so with the tacit approval of county housing administrators, who do little to encourage occupants to move on when their fortunes improve.