Before you finalise plans, book drain tracing and mapping to fix the route and depth, and a CCTV drain survey to check the pipe's condition.
Private drain or public sewer?
Everything depends on what the pipe under your proposed extension actually is. A drain carrying waste from your own property alone is generally private, and its owner is responsible for it. Once pipes from more than one property join, the shared length may be a public sewer owned by Scottish Water. In Scotland the line between the two is not drawn in the same place as in England, so a shared pipe running through a garden can be a public sewer even though it looks like an ordinary drain.
Scottish Water keeps records of its sewers and can supply asset plans, but older and smaller pipes are not always shown accurately. Tracing the drains on site settles which pipes run where before you commit to a design.
Building over or near a Scottish Water sewer
If a public sewer runs under or close to the footprint of an extension, conservatory or garage, you need Scottish Water's permission before work starts. Their concern is that the sewer can still be reached, maintained and replaced, and that the new foundations will not damage it. Depending on the pipe's size, depth and condition, they may agree to the work with conditions, ask for the sewer to be diverted at the applicant's cost, or require the building to stay a set distance away.
The application normally asks for drawings showing the sewer's position and depth, the proposed building and its foundations, and how access to the pipe will be kept. A camera survey of the sewer's condition is often requested so there is a record of the pipe before building work begins.
Building over a private drain
Where only a private drain is affected, the question is handled through the building warrant from your local council under the Scottish building standards. The drainage section of the technical handbooks covers how drains under buildings should be protected, how access must be maintained, and when a drain should be rerouted rather than built over. Your architect or designer will normally show the drains on the warrant drawings, so they need accurate information about where the pipes run.
- Inspection chambers should not end up sealed inside a room with no access
- Pipes passing under new walls or foundations need protecting
- Old, cracked or root-damaged pipes are best repaired or replaced first
- Moving a drain may be simpler than building over it
What to do before submitting plans
Find out early, before the design is fixed. Start with the title deeds and any old plans, then ask for a drain trace to mark exactly where each pipe runs and how deep it is. Have the pipes under the footprint inspected with a camera: building over a drain that is already cracked or full of roots stores up a costly problem underneath a new floor.
With the route, depth and condition known, your designer can decide whether to build over, bridge, divert or adjust the footprint, and you can approach Scottish Water or the council with drawings that match what is in the ground.
How long it takes and what affects the cost
Timescales depend on whether Scottish Water is involved, how complete your drawings are, and whether a diversion is needed. Allow time for it in your programme, because work over a public sewer should not start until permission is in place. The cost to the homeowner is mainly in the information needed: tracing, surveys and any diversion or repair work the approval depends on. Getting that information right first time avoids redesigns and repeat applications.
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Questions we get asked
Do I need a build over agreement in Scotland?
Not in the English sense. If your work affects a public sewer you need Scottish Water's approval; if it only affects a private drain, it is dealt with through the building warrant.
How do I know if there is a sewer under my garden?
Check your title deeds and Scottish Water's asset records, then have the drains traced on site, because records for smaller and older pipes are not always accurate.
Can I build an extension over a manhole?
Access to the drain must be kept. That can mean moving the chamber, rerouting the pipe or designing proper access, and it needs approval before work starts.
Should the drain be surveyed before building over it?
Yes. A camera survey shows whether the pipe is sound, so any repair can be done before it ends up under a new floor.