The brief
The clients had purchased a four-storey Victorian mid-terrace in the Royal Borough of Kensington and Chelsea. The property had a shallow vaulted cellar — roughly 1.8 metres at the crown, unusable as habitable space. They wanted a full basement excavation to create a large open-plan entertainment space with a home cinema, wine room, and utility area. The finished basement would add approximately 65 square metres of habitable floor area across the full footprint of the house.
RBKC has some of the strictest basement policies in London following its 2015 supplementary planning document. Single-storey basements under the existing footprint are generally supported, but the council requires a detailed Basement Impact Assessment, a Construction Management Plan, and evidence that the proposal will not adversely affect the hydrology or structural integrity of neighbouring properties. Our drawing fee of £1,950 covered the full architectural drawing set — the BIA and structural engineering were coordinated through our in-house team.
The challenge
Three factors made this project significantly more complex than a typical extension:
- Party Wall with two neighbours. As a mid-terrace property, both side walls are party walls. Under the Party Wall etc. Act 1996, any excavation within 3 metres of a neighbouring foundation (or 6 metres if the excavation line cuts below a 45-degree angle from the neighbour’s foundation) requires formal notices. Both neighbours appointed surveyors, and the party wall awards took eight weeks to agree before construction could begin.
- Waterproofing. The existing cellar showed evidence of lateral damp penetration. A Type C (drained) cavity drain membrane system was specified by the waterproofing engineer. Our drawings needed to integrate the cavity drain layout, sump pump positions, and drainage runs with the structural underpinning sequence designed by the structural engineer.
- RBKC basement policy compliance. The council required a Basement Impact Assessment covering hydrology, structural method statement, construction traffic management, and an arboricultural impact assessment for a London plane tree on the pavement outside. Our drawings and supporting documents had to demonstrate compliance with each of these requirements.
Our approach
We began with a detailed measured survey of all four floors plus the existing cellar, using a laser measure and digital level to map the existing foundation depths visible in the cellar. A trial pit was excavated by the contractor to confirm the foundation type and depth — as expected for a Victorian terrace, shallow strip foundations on London clay at approximately 900mm below ground level.
The drawing set included:
- Existing and proposed plans of all floors at 1:50, with the basement as a new sheet
- Two long sections and two cross-sections through the entire building at 1:50, showing the relationship between the new basement slab, the existing foundations, and the underpinning sequence
- Existing and proposed elevations (front and rear) at 1:100
- Lightwell detail drawings at 1:20, showing the new front lightwell with frameless glass balustrade and rear lightwell with access stair
- Waterproofing layout plan showing cavity drain membrane runs, perimeter channel, and dual sump pump positions
- Site plan and block plan at 1:200 and 1:1250
- Design and access statement addressing RBKC’s basement SPD requirements
The structural engineer designed a mass concrete underpinning sequence in six phases, working alternately under each party wall to maintain support at all times. Our architectural drawings coordinated with the structural drawings to ensure the finished floor-to-ceiling height of 2.6 metres was achieved after accounting for the structural slab depth, insulation, underfloor heating, and screed.
The result
RBKC validated the planning application within seven working days and the case officer recommended approval at week seven, subject to conditions covering construction hours, a Construction Management Plan, and a post-completion structural survey. No neighbour objections were received during the consultation period.
Construction took 10 months from the first underpinning phase to completion certificate. The finished basement provides 65 square metres of fully habitable space: a home cinema with acoustic treatment, a temperature-controlled wine room, a utility and plant room housing the sump pumps and mechanical ventilation, and a large open-plan entertaining area with polished concrete floors.
The project cost approximately £320,000 for construction (including structural underpinning, waterproofing, fit-out, and the lightwells) plus our drawing fee of £1,950 and structural engineering fees of £8,500. The clients’ estate agent estimated the basement added £400,000–450,000 to the property value.
