The Strip You Cannot Build On: Sewer Lines and Drainage Reserves

The architect has drawn a rear extension. It looks right, it works for the family, and everyone is happy with it. Then somebody asks where the sewer line runs, and half the drawing turns out to be sitting on a strip of land you are not allowed to build on.

This is one of the most expensive discoveries in a landed project, and what makes it expensive is not the constraint itself — it is finding out about it after the layout has been agreed, the budget set, and in the worst cases after a submission has gone in.

It is also entirely avoidable. The information exists, it can be obtained before a single line is drawn, and almost nobody asks for it early enough.

Two different things, regularly confused

People say "there is a drain running through the property" and mean one of two quite different situations. They have different rules and different consequences.

A public sewer crossing or bordering the land. This is a live public asset carrying somebody's sewage, buried at depth, and it does not belong to you even where it sits under your plot. Building a permanent structure over it is not permitted, and a clearance either side has to be kept — the figure is set by the requirements applying to that sewer, and it is obtained rather than assumed.

A drainage reserve or easement over part of the plot. This is a strip of your land reserved for drainage purposes. You own it; you cannot treat it as buildable. It often runs along a boundary or across a rear corner, and it is frequently the exact strip an owner had earmarked for the extension, the pool, or the covered car porch.

The practical difference matters. A sewer alignment sometimes has room for negotiation about what may sit above it and how close you may work; a reserve is a planning constraint on the shape of the buildable footprint.

Get the information before the layout is fixed, not after

Your Qualified Person obtains the sewerage and drainage information for the plot from PUB as part of the due diligence and submission process. It is a routine request. What is not routine is asking for it at the right moment.

The right moment is before the layout is settled — ideally before you buy, if you are still choosing between plots. The wrong moment is after the architect has developed a scheme, because then the information does not inform the design, it demolishes it.

When it arrives, do one thing with it that people skip: have it overlaid on the survey plan, at the same scale, as a single drawing. A written description of a sewer alignment and a separate site survey are two documents nobody reads together. One drawing with the constraint drawn on it is the thing the whole design team can actually work from.

It is worth doing this at the same time as the rest of the pre-design checks — the approval history of the property, what has been built before and whether it was sanctioned. We covered that side in reading the A&A approval history before buying, and the two searches belong in the same week.

What it actually costs you when it turns up late

The constraint is rarely fatal. What it does is move things, and moving things has a chain of consequences that owners underestimate.

• The footprint shrinks or shifts. The extension comes forward, or narrows, or the pool goes somewhere else.
• The structure changes with it. Move a wall line and you move a column line; move a column line and you move a pile. That is a structural redesign, not an eraser job.
• The envelope calculation changes. Setbacks, heights and the permitted envelope are all measured against a footprint that has just moved.
• Anything already submitted has to be revised. That is time, and on landed projects time is usually the thing the owner has least of, because they are paying to live somewhere else while the house is being built.

This is why a plot with a reserve across the rear is genuinely a different plot from the one next door, and why two houses on the same road can carry very different build costs for reasons that are invisible from the street.

Working near a public sewer

If the sewer is close but not under the proposed building, the design question becomes a construction question.

Excavation and piling near a live public sewer are treated seriously, because the consequence of damage is immediate, is somebody else's, and is not something you can quietly put right over a weekend. Depending on how close the works sit, protection measures, a method statement, or monitoring may be required, and the sequence of the works may be constrained in ways that affect the programme.

Two practical points that come out of this on real sites. First, the constraint often bites hardest on access rather than on the building itself — the strip you may not load with a piling rig or a crane outrigger is frequently the only place a machine could stand. That interacts directly with the access problem on a tight plot, which is its own subject: getting plant onto a narrow landed site.

Second, the position of a buried service is known to a tolerance, not to the millimetre. Where works are genuinely close, the alignment is confirmed on site before anything is dug rather than taken from a plan alone. That confirmation is cheap. Discovering the line by hitting it is not.

Your own sanitary line, and the chamber nobody thinks about

Everything above is about the public side. There is a private version of the same mistake, and it is more common.

Within the plot, the house's sanitary drainage runs from the building to the connection point, through inspection chambers. Building over your own line is not prohibited the way building over a public sewer is — which is exactly why it happens. It is still a bad idea, and here is the reason in one sentence: a blocked line under a new slab, a new driveway or a new tiled terrace is a demolition job.

So on any landed A&A, three things are worth insisting on:

1. Never build over an inspection chamber. Not slabbed over, not tiled in, not buried under a planter. If a chamber has to sit within a driveway or a paved area, it gets a proper access cover at the finished level, and its position is recorded.
2. Re-route rather than cover. If an extension lands on top of an existing run, divert the run as part of the works. It is a modest cost while the ground is already open and a very large one afterwards.
3. Record where everything ended up. Photograph the runs and the chambers before backfilling, with something in frame for position. The next owner, and quite possibly you in ten years, will need it.

In older estates there is a further wrinkle worth checking: lines do not always respect boundaries. A neighbour's run may cross your land, or yours theirs. That is a conversation to have while everyone is still friendly, not after a machine is on site — see also party walls and boundaries in terrace and semi-detached A&A works.

The order to do this in

1. Get the sewerage and drainage information for the plot through your QP, before the layout is settled.
2. Have it overlaid on the survey plan as one drawing.
3. Design the footprint around what that drawing shows, including the strip you cannot build on and the space a machine will need.
4. Where works fall close to a public sewer, agree the protection and the method before the programme is priced.
5. Deal with your own sanitary runs and chambers in the design, not on site.
6. Record everything before it is covered up.

The short version

A public sewer under the plot and a drainage reserve across it are different constraints with the same effect: part of your land is not buildable, and you need to know which part before an architect develops a scheme on it. The information is obtainable through your QP and costs almost nothing early. Overlay it on the survey. Design the footprint, the structure and the machine access around it together. And keep your own inspection chambers out from under new slabs, because that is the version of this mistake that owners make to themselves.

Planning an A&A or a rebuild and not sure what runs under the plot?

We build landed A&A and reconstruction work in Singapore as a BCA General Builder Class 2 (GB2) contractor, and we would rather look at the constraints before a layout is fixed than price a scheme that has to move. Tell us the property and what you are planning. Message us on WhatsApp, or see A&A works and in-house metal fabrication.