Will indemnity insurance solve this when I come to sell?

Partly, and less than people think. Indemnity insurance is a policy covering financial loss caused by a missing consent. It is quick, cheap, and solicitors accept it routinely. But it does not create the consent. It insures against loss if Thames Water acts, for example by needing access and damaging your structure. It does not guarantee a payout, and it may not satisfy every lender. It also has one hard limit: the policy is void the moment Thames Water is notified of the problem.

Can I get an agreement after the work is finished?

Sometimes. Thames Water's position is that a retrospective agreement may not be approved if the pipework was not properly protected to its specification when the work was done. It may require a CCTV survey, a relocated manhole, or remedial work before it will consider one. Here is the trap, and it is why this answer and the one above have to be read together. Applying for retrospective consent notifies Thames Water. Notifying Thames Water voids the indemnity policy. You have to choose a route, and most people discover that only after they have chosen.

Do I need one for a garden room, studio or shed?

Probably not, and that is worse news than it sounds. Thames Water's position is that a new structure not attached to the house, including a garden room, a shed or a car port, cannot be built over a public sewer at all. You have to apply to have the sewer diverted instead. A diversion is a bigger, slower and more expensive job than a build over agreement. The detached studio at the end of the garden is not the easy option people take it for.

Can the application run alongside planning, or does it hold up the start on site?

It can run in parallel with planning, and it should. It does not hold up the planning decision. What it holds up is the start on site. Thames Water requires the agreement to be in place before construction begins, because it may call for design changes. A design change after you hold a planning consent and a priced building contract is the expensive kind. Start the application early and run it alongside everything else.

I am buying a house that was extended and there is no agreement. What now?

This is how most people meet the problem, and in prime central London it usually surfaces on an unmodernised house that has already been extended once. You have three options: indemnity insurance, a retrospective agreement, or a CCTV survey and condition report to establish what is actually under the ground. They are not equivalent, and as the answer above explains, you cannot pursue all of them at once. Price the remedy into the offer, not into the regret.

Thames Water Build Over Agreements

What a build over agreement is, and who actually says no

A build over agreement is Thames Water's written permission to build within 3 metres of a public sewer, or within 1 metre of a public lateral drain. You get it from Thames Water, not from the council. It is a separate consent from planning permission, and here is the part most people get wrong: planning permission does not give you permission to build over a sewer.

Thames Water says exactly that on its own website. The assumption still runs the other way, all the way through the industry. People get planning, get building control, start work, and meet the third consent much later, usually through a solicitor.

A build over agreement is a contract with your water company, not a consent from your council. In London that company is Thames Water, the sewerage undertaker, meaning the company legally responsible for the public sewers in the region.

The trigger is distance. You need an agreement if you build within 3 metres of a public sewer, a pipe serving more than one property, or within 1 metre of a public lateral drain, the length of your own drain running beyond your boundary. Public means the water company owns the pipe, not you.

Thames Water puts the key point bluntly: "Planning permission does not grant you permission to build over a sewer or drain."

Building Control does have a role. Approved Document H4, part of the Building Regulations, requires a Full Plans application where you build within 3 metres of a mapped sewer. So the trigger does exist in the regulations, in black and white.

But Building Control cannot grant the agreement, because it does not own the pipe. Only Thames Water can say yes or no.

Two consents. Two bodies. Only one ever appears on a planning portal.

The pipe under your garden changed owner in 2011

On 1 October 2011, most private sewers and lateral drains in England and Wales stopped being private. Under the Water Industry (Schemes for Adoption of Private Sewers) Regulations 2011, they transferred to the water companies. In the Thames Water area this added roughly 40,000 kilometres of pipe to an existing network of about 67,000 kilometres, an increase of around 60 per cent, overnight.

Search for when build over agreements came in and you will be told 2011. That is the easy answer, and it is not quite true.

Control over building near public sewers is much older. Under the Public Health Act 1936, sewers already in existence on 1 October 1937 were public, and local authorities controlled building over them for decades. The agreement is not a 2011 invention.

What changed in 2011 was not the rule. It was which pipes the rule applies to.

A shared drainage run under a back garden, the sort of thing that was simply yours before October 2011, is in the ordinary case now a public sewer. Building over it needs somebody else's permission. The pipe changed owner, and nobody told the homeowner.

Three definitions follow, and everything else depends on them.

A drain serves one property and sits inside its boundary. It is private. It is yours.

A lateral drain is the length of that same drain beyond your boundary. Public since 2011.

A sewer serves more than one property. It is public where it connects to the network.

So the working test is short. If the pipe crosses your boundary, or serves your neighbour as well as you, it is probably not yours any more.

How to tell if there is one under your garden

There are four ways to find out, and you should use more than one.

The drainage and water search, the CON29DW, is the standard search a buyer's solicitor runs before a purchase. It reports what the water company's records show.

Thames Water's asset maps show the recorded position of the public network, and you can request them directly.

A CCTV drainage survey sends a camera down the pipe. It traces the actual run rather than the recorded one, and it is the only method here that tells you what is genuinely there.

The fourth needs no equipment. Ask whether the pipe serves more than one property, or crosses your boundary. If either answer is yes, it is probably public.

Now the honest part. Thames Water's own guidance says sewers commonly run beneath back gardens, particularly in urban areas. In Victorian and Georgian terraces, where houses went up in runs and drainage was shared from the start, the asset map is a starting point, not the truth. The records for this stock are incomplete, and a pipe that is not on a map is still a pipe.

So survey the line before anyone prices a foundation. A survey costs a fraction of a redesign. It is the difference between finding the sewer on a drawing and finding it with a digger.

The process, and what it actually costs

The cost depends on the pipe, and it starts at nothing.

For small, low-risk domestic sewers, where clearances are met and access points sit outside the building, you can self-certify. That is free. Everything else needs a full application, graded by pipe size.

Class 2 and above require a CCTV survey of the pipe before construction and another after. Class 3 adds surveys by approved contractors, and a deed of easement, a registered legal right for Thames Water to cross your land to reach the pipe.

On timing, Thames Water's service standard is that Class 1 and Class 2 agreements are usually issued within 21 days of receiving complete information. Read that as a service standard, not a guarantee, and note those last two words. For complex Class 3 work, Thames Water says the run from application to construction can take 3 to 9 months, sometimes longer.

One oddity is worth knowing. Thames Water's own class bands do not quite agree across its website, with different pipe diameters on the fee page and in the process text. If your pipe sits near a band boundary, ask which class applies rather than assuming.

Here is what it costs. These are Thames Water's own published figures, and a £130 discount is available on proof of entitlement. Charges are updated on 1 April each year, so check the current figure rather than any quoted in an article, this one included.

What Thames Water charges

Class	Fee
Self-certified	Free
Class 1	£130
Class 2	£490
Class 3	£585, plus legal costs starting at £1,000

Thames Water's own published figures. Charges are updated on 1 April each year.

What it does to the design

A build over agreement is not just paperwork. It changes the building.

Foundations have to bridge the pipe rather than bear on it. That means concrete lintels spanning the sewer, or deeper footings either side. It is a structural design problem, and it gets priced like one.

A manhole cannot end up inside the building. If one sits within the footprint of your extension, it moves. That is a cost and a programme item, not a detail to sort out on site.

Piling near a Thames Water asset requires a method statement setting out how the work avoids damaging it.

And if a build over is refused, the pipe moves instead of the wall. A diversion runs under section 185 of the Water Industry Act 1991, at your cost, starting with a refundable deposit of £2,000 for a minor one.

Now the local point, stated flatly. In Kensington, Chelsea, Mayfair, Belgravia and Marylebone the sewers are largely Victorian. They run under back gardens. The terraces share drainage runs, and those runs became public in 2011. Rear extensions and basements dig exactly where those pipes are.

That is not bad luck. It is the stock.

Basements sit in a separate lane, with their own planning-side assessment to a different body. A build over agreement is not a planning matter, and neither document does the other's job.

What happens if you skip it

Building over a public sewer without an agreement is not, in itself, illegal. That is the honest starting point, and it is why people get away with it for years without ever knowing.

What is true is that Thames Water keeps its powers over the pipe, whatever you put on top of it. Under the Water Industry Act 1991 it can maintain, repair and replace public sewers, and section 159 lets it come onto private land to reach one. Thames Water says that where building work limits that access, it could seek an injunction to remove the structure, and could recover damages.

The more common consequence is quieter. Building Control may withhold the completion certificate, the document confirming work meets the Building Regulations. Thames Water warns that without a build over agreement you may not be granted one, and that this can cause problems when you sell.

That is usually how people meet this. Not with a digger, not with an injunction. With a solicitor asking for a document that does not exist.

Two things are worth saying plainly.

The first is small and concrete. The fee for a Class 1 build over agreement is £130. It is published on Thames Water's website, free to read, in about a minute. Search for guidance on build over agreements and you will find guide after guide quoting different numbers, no two of them matching. An entire industry of advice about a £130 form, and almost nobody has checked the £130.

The second is bigger, and it is why this keeps happening. Nobody in the chain is lying to you. The planning consultant is right that it is not a planning matter, so they do not raise it. The architect cannot see a pipe that is not on the map. Building Control enforces H4 but cannot grant a consent it has no power to grant. Every one of them is technically correct, and the homeowner still ends up with an extension on a public sewer and no document to show for it.

Myrmex sits in the same chain. The only thing that catches this is somebody actually looking, before anyone prices a foundation.

Thames Water Build Over Agreements: The Consent Nobody Mentions