They also tend to know very little about drains and sewers

Have you actually established that these manhole covers are on public sewers? Just because there is a manhole, which may be for a drain, doesn't mean it is a public sewer.

My own experience, which was some years ago so things might have changed, is that 'building over' only relates to structures which are not readily removable to gain access to the sewer if it needs repair work. Digging through a patio to gain access to a pipe wouldn't be any different to digging through a driveway.

The thing that will upset water companies is if you cover a manhole, for example with paving slabs, even if the slabs aren't fixed in place with concrete. Locating a buried manhole, especially in an emergency is not an easy job, and due to the liabilities of flood damage, the water companies will want to be able to do that very quickly.

Another factor is the change in working techniques over the years. Unless the sewer is very shallow, then these days it is likely a repair will be carried out by a technique that doesn't involve digging - digging holes is fraught with health and safety issues, and the material dug out has to be disposed of which means landfill tax. A deepish public sewer will often be repaired by relining or 'pipe-bursting' from the nearest manhole. It is quicker and cheaper to do it that way, so digging up people's patios and driveways will be the last option chosen in many circumstances.

You also need to consider the cost of the work required. Assuming it was necessary to break up the patio to get to the sewer then £400 is going to cover a good proportion (perhaps all) the additional cost of the work that Thames Water may need to do in future because of the patio. You could pay out £400 to insure against a theoretical risk of work being done at some point in the future which may cost less than you are paying to get the insurance. (obviously you need to weigh up these risks when you make a decision)

The things I would do are:
1) Establish ownership of the drains/sewers (the local authority should have this information if the solicitor doesn't, and if they don't I would be considering changing solicitor!)
2) Find out the size of the pipe and the pipe depths (as above)

If the pipes are public sewers (owned by TW) and they are more than 5" (125mm) in diameter and are more than about 2 feet (600mm) deep then you will need to contact TW to confirm the situation if you remain concerned.

If they aren't owned by Thames Water then they probably belong to the house (and possibly neighbour) so you can lay whatever patios, paths, drives etc you like over the top of them. (but please keep the manholes clear )

Thank you - we know the drains are owned by TW, I am not sure on the size of them at this point.

The issue is that if we call TW and identify ourselves, we can't buy the insurance policy anyway as it would render it invalid (according to the Solicitors).

I think I will have to try calling again and present a hypothetical situation and see what they think.

You need to call them. You can give your name - they're trying to help you. It's the house you don't want to identify. Don't panic!!

Virtually all sewers are under ownership of the water companies now.

The only ones that are private are where your house is the first and/or only house on the run to the mains sewer, usually located in the street. You can tell because the first inspection chamber from your house on your land will only have one inlet - yours.

As soon as anyone's private drain crosses into someone else's land, they become 'public'. Often, if you can see three or more connections in your inspection chamber (neighbour's, yours and the pipe all the waste disappears off downstream in), they'll be public. You just need to trace where each branch comes and goes from.

Not really relevant to the OP as she has established the pipes are TW's, but a few extra bits of information to help anyone reading this in the future.

It is important to understand the difference between 'drain' and 'sewer' - they aren't the same thing. If you aren't sure which you have, check the definitions on a reliable website.

Most private sewers were transferred to the water companies, but that only affected the sections of pipework which are shared by more than one property. If you are the uppermost property on a run of pipes connecting to a main sewer, then the pipework in your property remains a private drain. However the same applies to the connecting pipes from the other houses to the shared (now public) sewer. If the pipe only serves one property then is is probably a private drain. Private drains only serving one property may become a 'lateral drain' when they cross a property boundary (e.g. into the road) when they become the responsibility of the water company.

There are exceptions to this. I think the definition is that the drain still has to 'communicate' with a public sewer for it to become public. So for example if your drain crosses into your neighbours garden and then into a ditch it will remain a private drain. The same applies to shared drains which don't communicate with a public sewer.

Unfortunatley the number of connections in an inspection chamber doesn't give a reliable guide to ownership. One household can have several pipes entering an inspection chamber. Especially in the 1930's it was common for drains from toilets to be kept separate from sinks and baths so multiple pipes could connect even to the first inspection chamber. It is also possible for a public sewer to have only one inlet and one outlet, and in many areas it was a requirement to have an 'interceptor chamber' before the connection to the main sewer in the road. If connected to more than one property these interceptors will be one in-one out inspection chambers on a public sewer. There is no hard and fast rule

I think I covered most of that in different language; talking about being the first or only house going to the mains sewer, using the word 'usually' and suggesting people trace back!

Thank you- it's crazy how the house buying process and Solicitor scare mongering can make you worry about everything!

They had me thinking originally that it was something special for the vendors to have confessed it to a solicitor, How else would they know?

I have in mind some foundations and a huge reinforced slab which a relative of mine was forced to install, when they discovered their car parking was sited over an ancient reservoir, long forgotten-about.

Hello all,

Thanks for all your replies.

I thought I would update this.

I called Thames Water and don't think that the sewer indemnity insurance is required as the foundations are less than 1m thick. I informed my Solicitor of this but they have said they will have to tell our mortgage lender that we are not taking out the insurance and this will push our exchange date back whilst they wait to see if the lender is ok with this. They also suggested that we are 'opening a can of worms' and that this may prompt our lender to look into other details of the property too, they may also decide that they will not allow us the mortgage unless we take out the insurance. The person I spoke to said that this is a risk they wouldn't personally take. I am not even sure what else they could look into?!

I now feel like I am in a position where I have no choice but to buy an insurance that we don't need. I realise that in the grand scheme of things the cost of this is a drop in the ocean compared to what we are spending but I can't help but resent spending over £350 on something irrelevant.

I guess we just have to get on with it!

I'm astonished!

I am glad I am not the only one who thinks this whole situation is bizarre. I can't understand how a patio affects lending...

They will only "look into" things which the solicitor and/or surveyor have highlighted to them - sounds like the solicitor has got a worm-can-opening production line ready to go!

I just can't really understand why they would want to do this.

I had offered to split it 50/50 with the seller in an effort just to get the exchange to happen next week. The seller has said they can't afford to do that as they are buying something (separate transaction). As if we aren't buying something too!

It really is bizarre.

Fair enough, if the vendor has built a closed structure within the presribed limit, as we thought ours had until the village water main was located, but a patio is just a bit of hard landscaping, like the roads under which sewers often reside.

Was it a specific comment in the survey that set this off?

I don't think so, the surveyor just said that there were manholes in the garden, I think she might have mentioned that she couldn't open one as it was stiff.

I feel like they have got me completely over a barrel, what is oddest of all, is that it is by my own Solicitor's!

Are you absolutely sure the manholes are on Thames Water sewers? I'm not asking because I am doubting what you say, but perhaps doubting what you have been told.

If the solicitor is so keen for you to take out the insurance and the person telling you the sewers are Thames Water's is the very same solicitor, then I would be triple checking this point. Have you actually seen in writing that they are in Thames Water's ownership, or is it only an opinion expressed by your solicitor? I'm not suggesting they are lying to you, just that they may have been misinformed.

If one of the manholes couldn't be lifted (not an unusual situation in itself) then nobody actually knows what that particular manhole cover does - it could be anything and not necessarily sewer or drain related. It could be covering a can of worms.

You might want to think about asking the vendor to get Thames Water to come and free up the stuck manhole to confirm what it is. You may find TW decide the sewers have nothing to do with them - often the case when there is the prospect of them having to do something, as opposed to them trying to stop you doing something.

Thank you for taking the time to write a reply.

I guess I can't be sure that they belong to Thames Water, that is just what the Solicitor has told me. I will try and do some research this weekend.

My husband thinks we may be cutting our noses off to spite our face and its now at the point where we should just buy the insurance. I just can't help feeling frustrated!

When I got my searches back (just north of Oxford), they showed the location of the public sewers (and, annoyingly, I may end up in a similar situation as the sewer just about overlaps with the extension - though the manhole is in the garden). It may be worth going through an searches your solicitor has sent you to work out if you have a similar map.

The quality and accuracy of what you get will depend on the local authority and the water company.

The original sewer records local authorities held may only contain main sewers in many cases as these made up the vast majority of the public sewer network. The exception were what was known as 'Section 24' sewers which were drains (usually on private land) serving more than one property and in existence before 1 January 1937. These were public non-main sewers. Depending on the authority, Section 24 sewers might be recorded on the official public sewer map, or they could be recorded in a written register. So not all public sewers may be recorded on the maps which local authorities hold or have access to for the purposes of searches.

There is an additional set of records held by building control, departments. These will variously shown the planned / as-built / modified drains within the property boundary - assuming the property was built or modified after record keeping began. These tend not to show who owns the sewer, and any information on a plan was only accurate at the time it was produced. Most private shared drains are now public sewers, but unless the water company has been very diligent these may not be mapped.

Another thing to bear in mind is how the sewer records have evolved over the years. In some areas the maps were kept on OS 1:1250 or 1:2500 scale plans - changes would be made manually. Some authorities arranged 'manhole surveys' with every manhole on the public sewer network being measured and sketched. This data was often eventually added to a computer database, in some cases with an OS grid reference scaled off a 1:1250 plan. This means the manhole position may only be recorded with an accuracy of around 10m.

The digital data that water companies now use for their mapping has either come from the manhole survey database, or by digitising the OS plan records, so in terms of position you shouldn't rely on these maps to be accurate, unless the water company has gone out and resurveyed the network using GPS.

So all of that was a rather long way of saying that if the plan you have shows the sewer just about overlapping an extension it could actually mean the sewer is under the extension, or maybe up to 20m away. Somewhere on the plan there should normally be a disclaimer along the lines that the positions shown are only approximate and should always be verified on site. Having a manhole in the garden might be a good sign though

Have to say that the village water main, which is plastic and not that old, was mapped wrongly, hence my worry about a new barn apparently built over it.

So I dowsed it, which was surprisingly easy to do, and features on the ground confirmed my 'crude' method of location. It's a good 6-8m from the barn!