Just in the process of buying a new house. Our plan is to build an conservatory across the back of the new house.

The search results from Thames Water indicate that there is a sewer pipe that runs across the patio exactly where we would want to build the conservatory. The accompanying solicitors letter says that, for an extension, we would need to seek as build-over agreement from Thames water.

Would this be required / an issue for a conservatory? Our intention is to have the it bricked right up to the roof on both sides, with a glass front and roof, so would be a reasonably heavy structure.

thanks

Yep. You’ll need a build over agreement. They might want to survey the sewer (at your cost) then charge you some admin fees. If the sewer needs work then that is their cost. Could be a few hundred for you. Not a huge amount over all but it all adds up. You could do nothing and just build it. If you don’t need building regs sign off they will likely never know but if something does go wrong you risk them demanding removal.

Thanks. That’s reassuring. Is there any reason you can think of that they may refuse permission? From the plan, it doesn’t look like there are any manhole covers on the property. I accept that I may have to pay for a survey and a bit of paper to show that I have permission.

Highly unlikely they’d refuse unless it’s some special sewer. They might insist on a particular foundation, usually that the footing goes deeper than the sewer and you bridge over it. Not too tricky as long as it isn’t massively deep. Domestic ones in garden are only 500mm to 1m or so deep so not a big deal.

That’s great. Thanks. May give Thames Water a call tomorrow to see if they think it may be an issue.

They sent us a letter recently – presumably they get a list of submissions from local authorities detailing planning applications or (in our case) building regs applications.

Conservatries are exempt from Regs so Thames Water might not find out if you don’t tell the LA.

Just done the same; £330 paid to the water company who came out twice, did camera work and went back to adjust their plans. Turned out the the pipe went under the house at a point under an existing wall, not the new one, but the planned amendment to the build was merely a concrete lintel of minimal span. Ironically, found another, unmarked one nearby, running the other way, so used the lintel over that one and kept schtum.

We (building regs) check the local water authority’s maps on receipt of all applications, some council’s may do it at planning stage. If a sewer is identified as likely to be within 3m of proposed works it needs a build over agreement. But, the build over requirements apply regardless of whether the structure is controllable under Building Regulations or not.

Cost for agreement with our water authority is nigh on £700 iirc.

They normally want foundations deeper than sewer, and shuttered around/bridged over. The cost of that for a conservatory, on top of the agreement may make the project very expensive, more than it’s worth in the case of a conservatory perhaps.

If you don’t need planning, or building regs, the water authority might not (probably won’t) ever be notified, and you could get away with it. BUT, when you come to sell the place, it WILL become an issue.

I went round this for my extension last year. The original plans had a 3m extension that put the back wall slap bang over the sewer pipe that ran exactly 3m away from my wall. The original plans had the sewer diverted around the extension but I didn’t like that as it seemed like an awful lot of digging and a man-hole cover at each junction/corner bend of the sewer meaning my patio being peppered with three or four manhole covers.

So I decided to build another 600mm out over the sewer – the local water authority (Severn Trent) preferred this option as it meant that the existing sewer was not disturbed so less risk of there being any problems created by the builders messing with the existing pipe, which they (the water company) would be liable for fixing.

As far as the agreement went no camera inspection was required and the builder did all the dealing with the water company (he does it all the time and knew everyone that worked there) so I had absolutely no involvement at all and the costs were negligible. However if they deemed a camera inspection to be necessary it would have cost me at least £350 to contract someone with the right kit to carry out the inspection and create the report. So I was quite lucky really.

We did this when building our extension.

Scottish water wanted:

Before and after camera surveys (£220 a go from contractor)

Foundations arranged such that the bottom was a concrete pad spreading the load over the top of the sewer (it was running parallel to the outside wall and too close to be bridged)

Overall it probably cost us about £800 or so once the extra foundations had been dealt with. I believe there is also a provision that they may have to ask us to demolish the extension in the event that they need access but IMO this is unlikely since it could easily be re-routed around it. Will look at the paperwork later.

And yes, building control wanted stuff like that sorted before going ahead.

Thanks. That’s reassuring. Is there any reason you can think of that they may refuse permission?

Because they can.

A friend lives next to the local Coop and has the same problem. Their lawyers point blank refuse to give permission so he can’t extend over it. Been arguing with them for years over this.

I had to get a build over for my extension as the sewer went directly under the extension. We had to have a supported steel been with a mesh rap. Calculations required cost a few grand extra. If it was my id say nowt.

If it was my id say nowt.

YANAL

Knowing what I know, I’d do it right. Fancy gambling your extension if they ever find out and have a need to get access?

Thanks all. food for thought. Bit cautious about not saying anything – even with no issues, as core says, it will come up on a search/survey when I inevitably come to sell.

If it was my id say nowt.

This is risky. It’s not particularly difficult to get an order to tear the thing down if Thames Water needs access. I’d also question whether the extension will be covered by buildings insurance. Oh, and good luck selling the house at a later date…guess what the buyer’s lawyer will look for as soon as they spot an extension built over a sewer.

I understand that there is an additional cost and frustration factor in getting TW’s agreement to build, but if you’re willing to take on the risks associated with doing it on the sly, you must really want a conservatory 🙂

Most water companies will allow construction over pipes up to 300mm dia nowadays. It’s a pretty simple process and only a few hundred quid and could save a hell of a lot of hassle in the future. Even minor works to small diameter pipes, like if you had to move a manhole to facilitate you’re extension is pretty easy as well.

Well for the water companies I’ve dealt with anyway, which are not Thames water btw.

It’s a pain, but definitely worth doing it legit as the potential consequences of building over without approval are really bad.

We had to bridge a public foul sewer on both sides of our extension, application cost was ~£400 (I think), survey costs before / after came to around £600, and Severn Trent insisted we repair a short section just inside the footprint of the extension. It was in a bad enough state to need repairing, but not bad enough that they would do it :o/ We replaced the entire run in the end, but that only added an extra £750 to the bill as the builder managed the contractors / timing very well to minimize costs.

The storm drain (3m deep) that ran parallel to the extension was another matter entirely, and caused a whole load of problems and expense.

We found that Severn Trent were very conservative when it comes to anything other than strip foundations, although the guy we dealt with was really helpful and worked directly with our builder + building control to come up with a satisfactory design.

Also might depend if its a public or private sewer – we ended up with a 3m easement on one domestic project because of that (no building over allowed)