Two Reston civic groups have issued a salvo of questions on a proposal by Fairfax County that could allow thousands more dwellings to be built in the planned community.

At the heart of the debate is how many people actually live in Reston, a carefully engineered suburban locale whose future is now being re-examined. County staff hold that fewer people live in each household than was first calculated when Reston was created decades ago. That means a density cap that governs the entire community, which hinges on that determination, could be reinterpreted to allow more construction.