Basement impact assessment in Merton

Merton ground and garden conditions

Merton is a borough of houses. 60.4 per cent of households live in one, 55.6 per cent own their home, and domestic gardens cover 27.2 per cent of the land in the national land-use database. That combination makes rear-garden basements and combined extension and basement schemes a regular brief.

Large gardens mean trees, and trees mean roots and moisture changes in clay. The ground investigation looks at them directly, because shrinking and swelling soil is a stability question for your own walls and for the neighbours'. We mark every significant tree on the plan so the report and the drawings tell the same story.

Neighbouring houses matter as much as your own. Terraced and semi-detached pairs share foundations at the party line, and the stability assessment has to show that the excavation will not undermine them. We record the depth and type of the neighbours' foundations where the investigation can reach them.

Heritage and listed buildings around a Merton basement

The borough has 28 conservation areas and 243 listed building entries, with 3 at Grade I, 3 scheduled monuments and 4 registered parks and gardens. In a conservation area the council expects a lightwell or a garden-level structure to respect the street.

23 Heritage at Risk Register entries sit within the borough too. For a scheme beside or beneath a protected building, the heritage case and the assessment go in together.

How Merton decides applications

The council decided 1,073 planning applications in its latest published year. It granted 92.9 per cent of householder applications and 82.5 per cent of minor ones. The timing figures are more modest: 85.2 per cent of householder cases and 82 per cent of minor cases were decided within the statutory period or an agreed extension.

With those timing figures, a complete first submission is worth the effort. Our planning drawings in Merton cover the set the assessment refers to, and the structural calculations in Merton page explains how the engineer's design meets ours.

Party wall and drainage rules for a Merton dig

Under the Party Wall etc. Act 1996, a notice is needed for excavation within 3 metres of an adjoining building that goes deeper than its foundations, and within 6 metres on the diagonal test. Serve it at least 1 month before work. Our party wall notices service prepares the plans.

Building Regulations then govern the floor. Where the water table can rise to within 0.25 metres of the lowest floor, it must be drained or protected.

Cost and scope for a Merton assessment

We are chartered architectural technologists (MCIAT). We coordinate the engineer, the ground investigation firm and the drainage designer, and assemble the pack so nothing on the list is missing. The price depends on depth, soil and tree cover, so after the free constraints check you receive a fixed quote.

Our order of work is the same on every Merton job. We read the policy, run the desk study, brief the ground investigation, then draw and engineer against the findings. The council sees one pack in which every number agrees, and you see a fixed price at the start rather than a series of extras.

If the scheme also adds a rear extension or a loft, say so now. The shared survey, the shared structural strategy and the single planning application save both time and fees.