Time phased causal factor

We have a scenario in which Time phased external factor impacts the real sales. For a internet service provider, forecast of connections will depend on new houses in the area. If the number of new houses are increasing swiftly then company expects to sell more of new Internet connections.

Client has the ability of finding the number of new houses in every region they operate and they know that number of houses will double in one of the region in Dec 2015, but forecast is not showing any jump in that time period with local causal factor because historical they were selling less. They are other regions where New houses number might grow by 25%