Hi Duncan - I had a similar problem as I too wanted to extend my railway from the back garden to the front. I couldn't go down one side of the house because of the boiler and the oil tank, the other side has a path between the garage and house and that wasn't practical. After spotting an air-brick in the wall and a little investigation I found it would be possible to take the railway under the house - so I did! It goes against all the normal rules for tunnels in our hobby; no longer than 2 arms-length long, easy access etc but I have a tunnel nearly 7 metres long and have had few mishaps in the ten years it has been in use. I have sufficient clearance for all my LGB locos and stock between the concrete foundations of the house and the joists that support the floor-boards, track is secured with alpine grit mixed with S.B.R bond (a bonding agent used in the building industry - it looks like PVA and dries clear) and tunnel portals have been fitted at either end.
I have followed your 'railway story' from the day you laid your first rails and I have enjoyed it very much - thank you.
Wistow Rail Rlwy.