If you specified to the builder it was required and he didn't do it and if you paid extra for it to be done then i would make him come back and do the job as you requested, especially if it was quoted in the price. His stuff up make him fix it, I would.
The only other option is go for a wireless network. Make sure it is 802.11g and set it to WPA or WPA2 if running windows XP and use a very long pass-phrase at least 12 letters or more. Also make sure the hardware is upgradeable by firmware updates. with some hardware i believe you can also reduce signal strength to prevent it going too far a field
Allen