My Upper East Side co-op building is considering adding a pool to the basement, at a cost of roughly $500,000. Realistically, how much value will this really add to our shares?

A building pool will add value to everyone's shares, but your board should do some careful research before taking the plunge, say our experts.

"On a ballpark basis, for an average building, I think the added value can be in the range of a five to 10% minimum increase," says Corcoran broker Deanna Kory, but a lot depends on factors including your building's size, location, and the apartments themselves.

If you're wondering whether the boost in property values will justify the expense of an assessment, Kory says, "it really only makes sense if it's a sizable building because then the cost can be spread out, and it's more likely that people will use it."

On top of that, she cautions, $500,000 seems like a low-cost estimate for this amount of work, so your board should further investigate the cost of the renovation, and consider other details like the pool's hours, whether there will be shower facilities or lockers, how much extra maintenance will be paid every month for upkeep, and what the plan is for an on-site lifeguard, as well as the building's liability.

Bottom line: A pool will probably be a boon to the building, but given the significant upfront and ongoing costs of having one installed, your board should have a careful plan in place -- and perhaps a second opinion from a broker or appraiser -- before moving forward.

Virginia K. Smith is the senior editor at BrickUnderground.com, the online survival guide to finding a NYC apartment and living happily ever after. To see more expert answers or to ask a real estate question, click here.