I took the 'growth in the area' as being a consequence of the Temple Farm development. This is also in Chelmsford.
They attract extra JW's into the area to build their headquarters for free.
They take advantage of the situation and spin this as growth and a need to provide a new place of worship. For some reason this is in a city centre office block - maybe something to do with the resale value and ease of change of use, but I don't know ;)
In the not too distant future...
Temple Farm is completed and there are no longer as many JW's in the area. There's no need for such a large meeting place. Watchtower puts it on the market as prime city centre real-estate and realises the profits.