Often committees are formed representing stakeholders, technical expertise, and the public who are asked to come up with a consensus forecast. Obviously, there is a political element to this as well as science. The ideal is that the forecast be unbiased and seek market efficiency. The composition of a committee and technical inputs to achieve good forecasts are matters for study, debate, and revision in light of experience. The best sources of forecasts for a municipal rule might well be the same private independent forecasting firms that are at present the best source of forecasts for private firms.
Developers buy forecasts, so can municipalities.
Property markets suffer a scarcity of institutions to produce unbiased forecasts and lack market information in general. The major way a permitting process might improve market information has to do with coordination on the supply side, regardless of which demand forecast is chosen. The present situation is that several forecasts are produced. Some developers may build excess space based on the more optimistic forecasts, given motivation to do so by fees and profit centres inherent in development, even when most in the market thinks there are already enough projects under construction. There is genuine supply side uncertainty due to a prisoner's dilemma game in office supply decisions.