Control door with RS232 or Ethernet relay
One cheap and wire-clutter free way to do it would be to use ESP8266 modules. They come in many flavors and ESP-07 is my favorite. This will allow you to control the door wirelessly over a wi-fi network.

There is a project which allows you to program ESP modules using Arduino IDE. This will get you up and running quickly. You will need the wiring diagram given on this page.

If you choose to use ESP-07, then you can safely use GPIO - 4, 5, 12, 13, 14 and 16. There are a few more but you need to be careful while using them. For your purpose six pins seem to be more than enough.

Once the ESP module is ready, you need to select a suitable relay. Better idea is to select a 5V coil relay (i'll explain why). Don't forget the freewheeling diode.

Powering everything up:

1) ESP requires 3.3 V for operation. If you have used 5V relay, then get a 5V adapter and use a cheap 3.3V linear regulator (1117-3v3 for example). If your relay used higher voltage such as 12V, the linear regulator would have gotten quite hot.

2) Relay can be powered directly using the 5V that is coming from the adapter.

Controlling the lock:

ESP can connect to your office wifi as a client. You can write a PHP application and send the signals to ESP using your web browser to control the lock.