Two ideals — more housing and sunny parks — are on a collision course in a San Francisco neighborhood primed for development. The city’s Recreation and Parks Commission is setting a possible precedent by blocking a small condo project that will shadow a prized outdoor parcel for nearly an hour on summer evenings.

The upshot is a win for a measure passed by voters in 1984 to protect parks against chilly shadows thrown by tall buildings. The decision will protect a basketball court and grassy hill at Victoria Manalo Draves Park at Sixth and Folsom streets.

It’s a modest and notable win for the South of Market neighborhood worried about losing ground to building plans around a popular gathering spot. But it raises another issue that underscores the problem with the motherhood issue of sunlight protection.

The rejected project totaled only nine units. But two others in the works total 92 and 116 units respectively, a sizable addition to the city’s costly housing market. These plans may also be stalled by the sunshine rule if a city commission decides the impacts are “substantial,” a decidedly vague standard.

The worth of the project, in this case housing, should be weighed against the loss of sunshine. It’s a choice that seems easy this time, but it could haunt the city’s larger interests in future votes.