From reader Jonathan Miller in New York Magazine, how to value a townhouse back yard: "What's outdoor space worth? Expect to pay $0.50 per sq ft of exterior space for every $1 of interior space if the exterior is functional. Add 5-10% for landscaping or a deck. If the exterior is large (50% or more of the interior), the math is this: $0.50 per $1 for the first 250 sq ft, $0.35 for the next 250 sq ft, and $0.10-$0.25 after that." Jonathan elaborates on the formula in a post on his blog, including a sketch of how to apply to a brownstone ground-floor apartment with garden.