Sequencing a basement excavation in a terraced street

A basement excavation terraced house project is judged as much on how the work will be done as on what the finished room looks like. The house either side is structurally tied to yours, the street is usually narrow, and every cubic metre of clay has to come out through the front door or a hole in the front garden. So councils condition the method, the hours, the lorry movements and the monitoring, and a scheme that arrives without a construction sequence gets asked for one.

Start with the sequence and the rest of the application follows.

Whether the basement needs permission at all

This depends on what you are doing. A basement wholly within the existing footprint with no external change is treated differently from one with a front lightwell, a rear extension below a garden, or an external staircase.

Where there is any external alteration, permission is normally needed. Where there is none, the position turns on whether the work amounts to an enlargement or alteration of the dwellinghouse, and councils take different views. Several London boroughs have removed the relevant permitted development rights by Article 4 direction, and many have adopted a basements policy in their local plan setting limits on depth, footprint and the proportion of the garden that can be excavated.

A Lawful Development Certificate is the only formal way to confirm that works are lawful without permission. This is general guidance rather than legal advice, so check the position with your local planning authority or a qualified professional before relying on it.

Why the sequence is a planning issue

Officers cannot assess structural design and would not try to. What they can assess is impact: on the neighbours' amenity during the works, on the highway, on trees, on groundwater and on the stability of the buildings either side. All of those are functions of the sequence rather than of the finished drawing.

Most authorities with a basements policy ask for a construction method statement and a Basement Impact Assessment at application stage, prepared by a chartered engineer. Councils usually expect a suitably qualified author for that assessment, and professional review is strongly advised, because it is the document that answers whether the neighbours' houses move.

The construction stages in a terrace

The order is fairly standard, and knowing it makes the rest of the application easier to write.

1. Ground investigation, party wall notices and a schedule of condition on the adjoining properties before anything is touched.
2. Temporary support to the existing structure, with needles and props where internal walls are affected.
3. Underpinning the party walls and external walls in short bays, cast in an alternating sequence so that only a small proportion of the wall is unsupported at any moment.
4. Bulk excavation within the completed perimeter, in stages, with temporary propping across the box.
5. Base slab, then permanent propping and waterproofing to the grade required for the intended use.
6. Internal fit-out, drainage and pumped connections, then reinstatement of the front garden or lightwell.

The alternating bay sequence is the part that governs the programme. Bays are typically around a metre wide and are cast in a pattern that leaves undisturbed ground between each excavated section, which is why supporting a party wall the length of a terraced house takes weeks rather than days. Attempts to speed it up by opening more bays at once are the classic cause of movement in the adjoining house.

Getting plant in and spoil out

Arithmetic decides this, and it is worth doing before the application. Illustrative example: a basement 5 metres by 12 metres excavated to 3 metres depth is 180 cubic metres in the ground. Clay bulks up when dug, so the volume to be carried away is closer to 230 cubic metres, and a grab lorry carries roughly 10 to 12 cubic metres. That is around 20 lorry movements for the excavation alone, on top of deliveries of concrete, reinforcement and props.

On a terraced street those movements need somewhere to happen. The usual arrangements are a conveyor from the excavation to a hopper at the front, a mini excavator lowered into the basement and left there until the end, and a grab lorry loading from a suspended parking bay. Where the street is too narrow, spoil goes out through the house in barrows and the programme lengthens considerably.

Set out in the application how many movements are expected, over what period, at what times of day and using which route. A Construction Traffic Management Plan is often conditioned anyway, and offering the detail up front removes an objection rather than answering one later.

Licences for the street

Planning permission does not give you any rights over the highway. Separate licences and permits come from the highway authority, usually the same council but a different department with its own timescales and fees:

• A skip permit for any skip on the highway.
• A scaffolding or hoarding licence where structures stand on the footway or carriageway.
• Suspension of parking bays for the grab lorry and the concrete pump, which usually needs several weeks of notice.
• A crane or oversail agreement where anything swings over the highway or a neighbour's land.
• A temporary traffic order where the street has to be closed, which takes longest of all.

Apply for these in parallel with the planning application, not after it. Bay suspensions and closures are the items most likely to delay a start on site.

Noise, hours and the neighbours

Construction hours are controlled by condition and by the council's environmental protection team. Typical permitted hours are weekday mornings through late afternoon with restricted Saturday working and none on Sundays or bank holidays, but the exact hours vary by authority and by street, so check the local standard.

For work of this intensity, a prior consent under section 61 of the Control of Pollution Act 1974 is worth considering. It fixes the agreed methods, plant and hours in advance and gives protection against a later noise abatement notice, provided the terms are followed. Without it, the council can serve a notice under section 60 at any point and dictate terms mid-programme.

The Party Wall etc. Act 1996 runs alongside all of this and is a separate legal process from planning. Notices, appointed surveyors, an award and a schedule of condition on the neighbouring properties are the mechanism by which damage is assessed and remedied. It is not a planning matter, and a council will not enforce it, but it is the main reason a well-run basement project stays out of dispute.

Monitoring while the work goes on

Monitoring is the condition householders least expect and the one that protects them most. A typical regime sets out survey points on the party walls and the neighbouring facades, baseline readings before work starts, readings at set intervals during the bay work and the bulk excavation, and trigger levels at which work stops and the engineer reviews.

Trigger levels are usually expressed in millimetres of vertical or horizontal movement, with an amber level prompting review and a red level stopping work. Groundwater monitoring may be added where the excavation approaches the water table. The results go to the council where a condition requires it, and to the party wall surveyors in any case.

Temporary works design is the discipline that keeps all of this safe, and the conditions councils place on temporary stability repay attention before the contractor is appointed rather than after.

Conditions that control the method

Expect some combination of a construction method statement to be approved before work starts, a construction traffic management plan, restricted working hours, wheel washing, a dust and noise regime, a monitoring scheme with trigger levels and reporting, tree protection where a street tree or a neighbour's tree is within influence, and a requirement that the works follow the approved sequence.

Read the conditions before you accept a contractor's programme. A programme that assumes continuous bay casting, six lorry movements a day and an eight to six working day is incompatible with most of the conditions above, and the gap becomes apparent in week two.

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