In the right conditions, 40 metres is no distance for 802.11. We ran a link over 200 metres at ~10Mbps, using two £20 Linksys Access Points and £12 antennas.
The problem with microwave signals is that they are reflected or absorbed by solid walls and vegetation. And the outcome is not easily predictable. Antenna theory is a nightmare. It's as much an art as a science. For example, the work that goes into calculating the best place to site a GSM/UMTS base station is phenomenal. There are so many factors to consider. e.g. when microwave signals pass over different ground conditions - such as pastureland and then over a road or a lake, the propagation properties vary enormously. When microwave signals bounce off buildings and other obstacles, it causes them to take multiple divergent paths, so you get "multi-path fading". There are then collisions where those multiple signals converge on the receiver in phase-shifted form, cancelling or "fading" the original signal. That generally is undesirable but Google for "MIMO" to see how it can even be exploited.
As for your own application, do you have line of sight between the two dwellings?
If so, with a pair of external parabolic dishes, or Yagis, coupled to two access points, one in each dwelling and configured in bridge mode, 40 metres is just a step.
Wifi hackers have successfully bridged the English Channel using off-the-shelf 802.11 kit coupled to highly directional antennas.
The American electronics supplier
Richardson Electronics (Rell) sells some enormous high-gain parabolic antennas for 802.11.. With beam widths of just a couple of degrees and gains in excess of 30dBi, these are intended for backhaul links of many miles. The antennas are surprisingly inexpensive. Around £60 for a 100cm dish. Obviously they are overkill for what you want, but Rell sells a wide range to suit all needs.
See
http://www.rell.comFor those on a tight budget, there is also
Stella Doradus in Waterford.. We bought some 45cm dishes with 15 dBi gain from them, and they were under €20. They exceeded our expectations.
See
http://www.stelladoradus.com/cheers, a