Wandsworth Thames Flats: Build-Over Agreements Explained

Wandsworth's riverside rebuild has produced some of London's most popular new flats. It has also produced some of its trickiest drainage. Under the glossy towers and converted wharves runs a network of public sewers that Thames Water still owns. When anyone plans to build above or near those sewers, a build-over agreement becomes essential. This article explains what it involves, why Thames-side homes carry extra risk, and what to do before your lawyer exchanges contracts.

What Is a Build-Over Agreement?

A build-over agreement is formal consent from Thames Water to build close to a public sewer, or right above one. Without it, a building over a public sewer may be unlawful. You could even have to remove it at your own cost if Thames Water needs access for repairs.

The Water Industry Act 1991 gives water firms the right to reach and maintain public sewers. If your extension, cellar or garden room sits over a sewer and blocks access, Thames Water can serve a notice to remove it. This happens more often than people expect in older parts of London, since many owners never knew a public sewer ran under their home.

Why Wandsworth Thames-Side Properties Are High-Risk

The riverside has seen heavy rebuilding in recent decades. The sewers under these sites date partly from Victorian times and partly from later work. Several things make build-over cases common here.

Old industry sites. Former wharves, gasworks and factories had their own drains. Many joined the public sewer network when the land was cleared, so their routes are not always clear on standard sewer maps.

Cellar and undercroft parking. Many blocks have cellar parking or plant rooms at or below sewer level. Any load-bearing wall at that depth needs a careful check by engineers, because it sits next to the sewer.

Planning myths. Some flat owners assume that council approval means Thames Water has agreed too. Yet planning permission and build-over consent are separate. One does not imply the other.

Shared ownership issues. In a leasehold block, the duty to get consent may fall on the landlord, the block manager or the leaseholder, depending on the work. Older leases are often unclear on this.

Our pre-purchase drain survey can find hidden build-over cases in Wandsworth. They often come to light during the sale, since the buyer's lawyer runs a drain search.

The Build-Over Agreement Process with Thames Water

Thames Water runs a set process. In broad terms, the steps are as follows.

Step 1: Drainage search. Your lawyer orders a CON29DW search as part of the sale. It shows whether a public sewer runs within the boundary, and it flags any existing agreements or legal notices.

Step 2: CCTV survey. Thames Water asks for a CCTV survey of the sewer before and after the build. The first survey records the sewer's condition, so anyone can spot damage from the work later. Our CCTV drain surveys are often booked for this, usually at the request of a structural engineer or lawyer.

Step 3: Application and drawings. The owner or their agent sends in detailed drawings. These show the new work, how close it is to the sewer and the foundation design. Thames Water then checks the loads and the access plans.

Step 4: The agreement. If it agrees, Thames Water issues a formal build-over agreement. This is a legal paper that stays with the title, so future buyers must hear about it. Hiding it is a serious fault.

What Happens if There's No Agreement in Place?

Buyers of Wandsworth riverside homes must watch for this. If a past owner built an extension, deck, garden room or cellar with no consent, you inherit the problem. Lawyers call a missing agreement a latent defect. It is hidden, yet it is yours from the day you buy.

The results can be serious.

• Thames Water may ask you to remove the work if it needs sewer access
• Your lender may refuse to lend without late consent or cover
• You may struggle to sell until you settle the issue
• If the sewer was damaged during the build, and nobody surveyed it first, you may face a repair bill

Late consent is sometimes possible. However, Thames Water will want a survey after the build, and it may set conditions. In the worst cases, the sewer is damaged or access is blocked, so it refuses consent and sends removal notices.

What to Do as a Buyer of a Wandsworth Flat

Here is our advice for buyers of riverside and former industry conversions in Wandsworth.

Commission a pre-purchase drain survey. Before you exchange, arrange a pre-purchase drain survey. It should cover the private drains and any public sewer nearby. The report shows whether a public sewer is there, what state it is in, and whether any build-over cases exist.

Check the CON29DW carefully. If the search shows a public sewer close to any part of the building, ask your lawyer to confirm an agreement is in place. If there is none, negotiate a retention or an indemnity.

Do not assume you can build freely. If you plan to build later, such as a garden room, a deck or a loft conversion, check the sewer position first. Our Wandsworth drainage team can check it before you hire an architect.

Understand your lease. In a leasehold block, read who must get consents for shared drains. If the landlord should have got consent for the cellar car park and did not, the fault is theirs. Yet it will still affect your ability to sell.

Costs Involved

The price of a build-over application depends on the sewer and the scope of the work. The surveys before and after add to it. These costs are modest, because they prevent delays to the sale and legal action.

Late consent often costs more, since it involves extra legal letters, and the lender may ask for cover.

Frequently Asked Questions

Does every flat in a Thames-side development need its own build-over agreement?

Not individually. The original builder usually gets consent for the whole site, and the agreement stays with it. Yet any later work by a flat owner may need separate consent if it affects the sewer.

How do I find out if there's a public sewer under my Wandsworth property?

The CON29DW drain search shows public sewers registered with Thames Water. However, sewer maps are not always accurate for older pipes. A CCTV survey plus the drain search gives the most reliable picture.

What is the difference between a build-over agreement and a sewer adoption?

A build-over agreement covers building near or over a public sewer. Sewer adoption is a different process, where a private sewer becomes public. Both matter on Wandsworth's former industry sites, since many private sewers were adopted under the 2011 private sewer transfer regulations.

Can I get indemnity insurance instead of a build-over agreement?

Sometimes. Indemnity cover protects you from the cost of legal action, yet it does not fix the legal issue. Lenders differ, and some insist on late consent regardless. Your lawyer should advise on your lender's rules.

Can you carry out a pre-purchase drain survey in Wandsworth?

We cover Wandsworth for CCTV drain surveys. Request a quote online via our contact form or visit our Wandsworth drainage page to arrange a survey that fits your timetable.