Is the construction traffic plan part of the assessment, or a separate document?

It is separate. Alongside the engineer's report, a basement application in Kensington and Chelsea must include a draft Construction Traffic Management Plan, or CTMP, which sets out how lorries, deliveries and parking suspensions on the street will be managed. The council secures the full version through a planning condition once permission is granted. It sits next to the Construction Method Statement, not inside it.

Do I need a party wall agreement as well as planning permission?

Usually yes, and it is a separate process. Planning permission and the Party Wall etc. Act 1996 are different things. The Party Wall Act is a civil process between you and your neighbour, covering the shared wall, the works and any damage. It runs in parallel with planning, and it neither grants nor blocks planning permission.

Is structural sign-off part of the planning assessment?

No. Planning controls land use and the impact of the development. The structural stability of the works themselves is controlled separately, through Building Regulations, which are the national safety rules for how a building is constructed. The two run on different tracks, so planning permission does not sign off the engineering.

Why does the council care about other basements near mine?

Because the effects add up. The council treats the cumulative impact of several basements in one area as a planning consideration in its own right, for example on groundwater and ground movement. It does not judge each dig in isolation. A scheme that looks harmless on its own can matter when set against its neighbours.

Can I build a basement if my home is listed or in a conservation area?

It depends which one. Excavation underneath a listed building, including its vaults, is not permitted. A basement in a conservation area is treated differently: one with no visible external elements is not, by itself, treated as harming the conservation area. So the two situations have very different answers.

Basement Impact Assessment in Kensington & Chelsea

What a basement impact assessment actually is

The phrase you typed is borough-specific, and that is the first thing to understand. Some London councils, such as Camden, have a formal, audited document literally called a Basement Impact Assessment. Kensington and Chelsea does not use that name at all. In the Royal Borough, the same job is done by a Construction Method Statement, submitted under the council's basements policy.

A basement impact assessment is a technical report, submitted with a planning application, that sets out how a proposed basement will affect the things around it. It covers the structural stability of your own building and the neighbouring ones, the ground itself, water in all its forms (groundwater, surface water, drainage and flooding), and nearby trees. Because it spans structure, ground, water and trees, it usually draws on more than one specialist, not the architect alone.

The exact name for it varies by council, which is where the confusion starts. In Camden, Basement Impact Assessment is a defined, audited document with that precise title. Kensington and Chelsea does not use the phrase at all. In the Royal Borough, the same content sits inside a Construction Method Statement, usually shortened to CMS, which is the engineer's report describing how the basement will be built and what effect it will have.

The CMS is required under Local Plan Policy CD11, formerly Policy CL7, and the more detailed Basements Supplementary Planning Document, or SPD, which is the council's adopted guidance that expands on the policy. The SPD was adopted in April 2016 and remains the operative detail. So when someone in Kensington and Chelsea says basement impact assessment, the document they actually need is a CMS.

Do you need planning permission for a basement in Kensington and Chelsea?

Yes, always. Every basement in Kensington and Chelsea now needs planning permission, with no exceptions and no automatic right to build one.

This was not always the case. Some building works fall under permitted development, which is the set of works you can normally carry out without a planning application. Basements used to sit partly within that category. That is why an older neighbour may have built one without a planning application, and why their experience is no longer a guide to yours. The council removed them from it through a borough-wide Article 4 Direction, which is a council order that switches off permitted-development rights for a defined type of work in a defined area.

The dates are settled. The direction was made on 15 April 2015, confirmed on 2 March 2016, and came into force on 28 April 2016. Since then, the permitted-development route for basements has been closed across the whole borough, and every basement needs a full planning application. In practice that means drawings, the engineer's report, and the council's assessment before any digging can start.

If you want the wider picture of how permitted development differs from full planning permission, that is a separate question worth understanding before you start.

What the assessment has to contain

The Construction Method Statement has to do one thing above all: show that the basement can be built without damaging your building or your neighbours'. To do that, it sets out a defined list of contents.

It must include a non-technical executive summary, a plain-language overview for people who are not engineers. It must include a desk study and a site-specific ground investigation, which is testing the actual ground under your house, including monitoring the groundwater. Real testing matters here, because assumptions about what lies under a London street are often wrong. It must describe the structure and foundations of your existing building and of the relevant neighbouring buildings.

It must assess the basement's impact on groundwater. This is the hydrogeology, which means how water behaves underground. Under much of the borough, water sits in a layer of gravel above impermeable London clay, so digging into it can disturb how the water drains. The report must also cover surface water and a Sustainable Drainage System, or SuDS, which means managing rainwater on the site so the basement does not add to flood risk. And it must include a flood risk assessment, which judges the chance of flooding and how to manage it.

It must set out the construction sequence and the temporary works, which are the propping and supports that hold everything up during the dig. It must predict ground movement and give a damage-category assessment for the surrounding buildings, meaning the engineer predicts how far the ground will move and rates the likely level of damage next door. And it must address tree protection where relevant, including root protection areas, the zones around a tree that must be left undisturbed, to the British Standard BS 5837, published in 2012.

One requirement sits above the rest. The CMS must be signed by a chartered engineer that the applicant appoints, and it must conclude that the design meets the structural-stability criterion in the policy. The Basements SPD sets this out. The engineer must be a Chartered Civil Engineer, holding the MICE qualification, or a Chartered Structural Engineer, holding MIStructE.

What you are allowed to build

What you can build is capped by Policy CD11, and your CMS has to show the design meets those caps. People often confuse what you submit with what you can build, so here are the limits the design must respect.

A basement can cover no more than 50% of each garden or open part of the site. That is a reduction from the former limit of 85%, so older schemes are not a guide. A basement can be no more than one storey deep, which is roughly three to four metres from floor to ceiling, with exceptions only on large sites. There must be at least one metre of soil above any basement built under a garden, so that planting can survive above it. These limits exist to protect gardens, drainage and the structure of the wider terrace, not just your own plot.

The Sustainable Drainage System must be kept in place after the work is finished, not just installed and forgotten. You cannot add an extra basement floor where a basement already exists, or where one was built under the old permitted-development rules, so stacked basements are out. The rule looks at the building as it stands now, including anything already dug beneath it.

And there is a firm heritage limit. You cannot excavate underneath a listed building, which is a building on the national list of those with special architectural or historic interest. That prohibition includes its vaults. Listed building consent and planning permission are separate consents, and this limit sits on top of both.

If your property is in Westminster

If your home is in Mayfair, Belgravia or Marylebone, it sits in the City of Westminster, a different council with its own rules. Westminster controls basements through City Plan Policy 45, and it too removed permitted-development rights for basements by Article 4 Direction, in force since July 2016, so permission is always needed there as well. Instead of a CMS, Westminster requires a Structural Methodology Statement, self-certified by a chartered engineer of the same standing, along with a soil investigation, a geo-hydrology assessment and a SuDS statement where the excavation is substantial. Policy 45 limits a basement to a single storey below the lowest original floor, and to 50% of the garden.

The part that is not paperwork

A basement impact assessment is sold as paperwork. It is not paperwork. It is the single document in which a named engineer puts their name to a specific claim: that digging a large hole under your house will not crack the house next door.

That is worth sitting with. In Kensington and Chelsea, the council independently audits that engineering only in limited cases. So the safety of the most invasive thing you can do to a London house rests on the competence of the engineer the owner chose and paid for. Much of the industry, Myrmex included, is content to present this as a box to tick. It is not a box to tick. It is the centre of the whole project.

One assessment also pulls together a lot of separate threads: an engineer, a hydrologist, an arborist, a traffic plan, a planning submission and a party-wall process, each with its own timetable. Getting them to line up is the real work. If you want to see how a basement project is coordinated from the start, that is the next thing to read.

This is general information, not structural, legal or planning advice. A construction manager coordinates the project; it does not replace the structural engineer who signs the assessment, the solicitor who handles the legal side, or the planning consultant who runs the application. Get those judgements from the people qualified to give them.

What Is a Basement Impact Assessment?