Well considering you can't have negative vacancy rates...
And it's not healthy for a market to have a rate that low anyway, so it's not something that should be striven for.
Regina has a Class A vacancy rate for its competitive office market of 0.86% (not healthy), and an overall rate of 1.8% in the competitive office market. If you include the non-competitive market (company owned buildings, Government occupied, long-term leases), the overall rate drops down to 1.01% out of about 6.3 million sq ft of office space.
This is why there is a tower currently under construction, one in the final planning/proposal stages, another one or two rumoured to be in the works, and a shorter building downtown currently in the prepping stages to provide some Class B spaces for smaller companies.
With all of these developments, the market might finally approach a vacancy rate that's healthy... but who knows, with the way the economy is, it's anyone's guess as to how quickly it will be absorbed.