I am building a new office next to the house; it is about 1.5m away. I need to run, from the house's upstairs bathroom, 1 hot, 1 cold, and radiator with return (all of which I believe are 28mm to the bathroom).

They need to come down the outside wall (2.5m ish), under the path (1.5m ish) and then back up to the office (0.5m ish). My problem is making sure they are both insulated for heat lose and frost damage (south of England, -5°C is about the worst).

I was going to go out of the bathroom into a pipe (something like a 110mm waste); in this pipe I was going to use pieces of wood to brace the pipes in the middle of the pipe (think of a large button that would wedge itself in the waste pipe and the water pipes run through the thread holes).

I was going to use some kind of plastic pipe (what is best?). I was also going to wrap rubber or foam around each pipe, and to top it off, fill the voids of the "waste" pipe with foam chips or beads.

In case of upcoming long nasty periods and the building not being in use, I was going to put stop valves (with stop and "bridging" valves for radiators) in the house bathroom and then draining valves in the office. In fact, I'm not sure I will ever do this as I do not want to do the massive bleeding task after, probably easier just to keep on low heat all winter.

Does this sound like a viable idea? Has anyone done something similar or have another (better) solution?

I enjoy plumbing but I am in no way trained, so…

  • Where are you located? The biggest obstacle is keeping your pipes from freezing.
    – DMoore
    Commented Jul 4, 2013 at 19:18
  • @DMoore South East of UK, the lowest on record is -13oC I believe. The underground bit is okay (will just put below frost line) but the pipes running down the house are a problem. I could put them inside but it is massive job (making wholes through underfloor heating, etc). Other option is to tap off the mains cold inlet and then run a smaller boiler in there which would be a shame as the one in the main house is supper efficient heat exchange thing. Commented Jul 5, 2013 at 10:35
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    You are going to have to punch holes in your foundation to run the lines. There is really no choice unless you want to check on this daily when it is cold. I would run them next to each other and wrap them.
    – DMoore
    Commented Jul 5, 2013 at 21:21
  • @DMoore Really... What a pain, the foundations are only a year old and are massive and as I say it also means going through underfloor heating pipes/screed, etc. I am thinking local boiler now as that is such a big job. Thanks for advice Commented Jul 8, 2013 at 13:21
  • DMoore is correct, these lines will have to go through a foundation wall.
    – Handy Man
    Commented Jul 15, 2014 at 17:59

1 Answer 1


This is a very localized question. If this situation happened in Africa it would be perfectly fine to run the line straight to other house. The whole point is to have the pipes running through an area that won't freeze.

It will really be up to your local building code. You want to ask your local inspectors. I have set up plumbing for exterior garages in the midwest and I have been told between 2-3 feet deep. But I am not sure they cared that much since everything external I have done was garage related. The whole point is to get the water lines below the freeze line. I would think that the UK would have similar requirements as midwest US (maybe we are colder for longer periods).

  • Thanks, I am putting in planning soon for the annex so I am sure I will be told then. I suppose if it is deeper then the frost line then it can be as long as needed, in which case I could run it around the side of the house and directly into the boiler. Commented Jul 29, 2013 at 9:26
  • The U.K. has much milder weather than the U.S. midwest. The average low temperature is never below freezing. See this.
    – wallyk
    Commented Jul 15, 2014 at 21:50

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