Underpinning a terrace: what planning does not control

Underpinning an ordinary terraced house does not normally need planning permission. Section 55(2)(a) of the Town and Country Planning Act 1990 takes works of maintenance, improvement or alteration out of the definition of development where they affect only the interior of the building or do not materially affect its external appearance. Underpinning is below ground, so on most houses it clears that test comfortably.

That answer changes if the building is listed, and it changes if the underpinning is bound up with excavation for a basement. The other consents run regardless.

Whether underpinning is development at all

The test in section 55(2)(a) has two limbs and only one has to be satisfied. Works affecting only the interior are excluded. So are works that do not materially affect the external appearance.

Underpinning generally satisfies the second limb. The mass concrete or beam and base is below ground level, and once the ground is reinstated there is nothing visible from the street. Where the work exposes and later reinstates a front garden, the appearance is restored.

Two situations change the answer. Where underpinning is part of a scheme to lower the floor level and create a basement, the project as a whole may involve excavation, new lightwells, external stairs or changes in ground level, and those are development. And where the work raises the ground level, alters a retaining structure visible from the highway or removes a boundary wall permanently, the external appearance is affected.

If there is doubt, the only formal way to settle it is a certificate of lawfulness for proposed use or development under section 192 of the 1990 Act. It costs less than a refused retrospective application and it is the document a buyer's solicitor will want.

Listed buildings and the separate consent

For a listed building the position reverses. Listed building consent under section 7 of the Planning (Listed Buildings and Conservation Areas) Act 1990 is required for works that affect the character of the building as a building of special architectural or historic interest, and that expressly includes internal and structural works.

Underpinning a listed building will normally need consent. It affects historic fabric at foundation level, it can alter the way the building moves and breathes, and the introduction of rigid concrete beneath a lime-built structure has consequences that a conservation officer will want to understand.

What is expected with the application: a heritage statement describing the significance of the building and assessing the effect of the works, a structural report explaining why underpinning is the right remedy rather than a lesser intervention, and a specification that addresses materials and their compatibility with the existing construction.

Carrying out works to a listed building without consent is a criminal offence, and the fact that the works are below ground is no defence. In a conservation area, by contrast, underpinning an unlisted building needs no separate consent unless it involves demolition.

Works affecting a shared structure

On a terrace, underpinning almost always engages the Party Wall etc. Act 1996, and this is the process people underestimate.

Two sections usually apply. Section 2 covers works to a party structure, including underpinning a party wall. Section 6 covers excavation within three metres of a neighbouring structure to a depth below its foundations, and within six metres where the excavation cuts a line drawn at 45 degrees down from the neighbour's foundation.

The procedure runs on statutory notice periods, two months for party structure works and one month for excavation notices. If the adjoining owner dissents or does not respond, surveyors are appointed and an award is made, which takes longer again. Building before the award is in place is a breach and can be injuncted.

None of this is planning. The council will not check it, and a planning permission does not authorise it. But it sits squarely on the critical path, so serve notice as soon as the design is settled rather than when the contractor is booked.

A structural survey recording the condition of the adjoining properties before work starts protects both sides, and party wall surveyors will usually require a schedule of condition in any event.

Building control and the evidence a council still wants

Underpinning is building work under the Building Regulations 2010, so it requires a building notice or a full plans application, and it will be inspected. That is the consent that actually controls how the work is done.

The design will normally be supported by a ground investigation, because the remedy depends on what is causing the movement. Clay shrinkage from a nearby tree, a leaking drain, made ground and mining legacy each point to different solutions, and underpinning the wrong problem is expensive. A geotechnical assessment sets out the ground conditions, and where a tree is implicated, the answer may be drain repair and tree management rather than underpinning at all.

Even where planning permission is not required, the local authority may take an interest in two situations: where a tree contributing to the cause is protected, so removing it needs consent; and where the works involve a highway licence, hoarding or a skip permit.

Note: underpinning that changes the structural relationship between two halves of a semi-detached pair can cause differential movement. That is a design issue, not a consent issue, and it is where an experienced structural engineer earns their fee.

Insurance-led works and their sequencing

Most domestic underpinning follows a subsidence claim, and the insurer's process sets the sequence.

The usual order is: a claim, an initial inspection, a period of monitoring often running through a full seasonal cycle, a site investigation with trial pits and soil sampling, a report identifying the cause, then a schedule of works. Monitoring is what takes the time, and it is not padding. Distinguishing seasonal clay movement from progressive settlement needs a year of readings.

Where vegetation is the cause, the remedy is often management or removal rather than underpinning, which is cheaper and less disruptive. Where a drain is the cause, repair may be enough. Underpinning is the last resort, and an insurer who proposes it early is worth questioning.

Coordination matters when the property is one of a pair. Underpinning one half and not the other creates a stiffness difference at the party wall, so the engineer should consider the terrace as a whole even when the claim covers one house.

Recording what was done for a future sale

Subsidence history follows a property, so the paperwork is part of the asset.

Keep the structural engineer's report identifying the cause, the design and calculations, the building control completion certificate, the party wall award and the schedule of condition, photographs of the works in progress, the guarantee or warranty from the contractor and its transferability terms, and any post-works monitoring records showing movement has stopped.

Buyers' solicitors ask for all of this, and lenders' valuers ask for the completion certificate and the engineer's sign-off. A property with a complete file sells at a different price from one where the owner remembers that some work was done in 2019.

Where the works were carried out on a listed building, keep the listed building consent and the discharge of any conditions with the file.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.