Optical
If you are fortunate enough to have line of sight to your new building and the distance is less than about 1.2 miles, a laser-based solution is your fastest option. Sporting speeds up to 1.2 Gbps and good security (it's hard to tap into a beam of light), these systems can be ideal for your needs. The process in setting one up is to mount the units (one on a building attached to your network and one at your destination building), align the beams, and go. The drawback is inconsistency due to weather conditions or wind, which might knock your alignment off. However, this is mitigated by auto-tracking features on many models that help keep the beams aligned.
Microwave
Although less strict than a laser, a line of sight to your building will again be required to employ this wireless bridge solution; however, your reach is extended to about 25 to 35 miles. Unfortunately, while the speeds have increased from the 10-Mbps setup that I used years ago, you won’t be getting Gigabit speeds with microwave. If you can handle speeds in the 10- to 90-Mbps range (depending on your equipment), microwave can be an excellent solution to deploy. Easy to set up, easy to maintain, and fairly inexpensive -- roughly $10,000 (£5500) to $12,000 (£6600) per installation -- this setup can pay for itself in a year and a half. Again, the weather can cause some inconsistency, but usually only in extreme cases. Wind can affect the alignment, but again, assuming it was installed correctly, it will take a pretty strong wind to require reconfiguration.
RF (Radio Frequency)
If perfect line of sight is a problem, or you need to go a significant distance, than RF may be the choice for you. Boasting speeds of up to 300 Mbps to a distance of 100 miles or so, and with the ability of some equipment to 'bend' signals around obstacles, RF can be the long-haul solution in your metropolitan network. RF is very weather resistant; however, security can be an issue. After all, it is a broadcast radio signal, so VPN is a requirement over this kind of connection. Lastly, RF can interfere with other RF, so if there are a lot of other transmitters in the vicinity on the same frequency, there can be some interference.
While none of these solutions are perfect, they may be the best alternative to a WAN connection, depending on your situation. The prices are competitive compared with a leased line or frame relay connection over a year or two and give you the added bonus of moving it and using it later should the department or organisation you are supporting relocate again.
