When adding a new floor is not enough: extending your basement laterally under the garden, side return or front pavement vault to unlock a full-size London lower ground floor. Directly-employed team, 25-year warranty.
150+ basements delivered · Considerate Constructors members · Free site visit and feasibility read.
A basement extension goes beyond a straight underpin of the existing footprint. It excavates and structures new below-ground space under the garden, the side return, the front pavement vault or a rear extension, creating a lower ground floor bigger than the house above. In London, where planning above ground is tight and gardens are precious, it is often the only way to double the usable footprint of a family home.
A structural basement project extending the below-ground envelope beyond the existing footprint, planned with your architect and engineer, delivered by our directly-employed team, warranted for 25 years.
Homeowners in London who need substantially more space than a straight under-house dig-out delivers, typically a full open-plan kitchen-diner with garden light wells, or a self-contained lower-ground suite.
When planning above ground is constrained, when garden space is precious, or when a straight excavation under the existing footprint doesn't deliver the plan you actually want.
Basement extension is where the plan you dream of becomes the plan you can build. It also demands more sophisticated planning, structural design and party wall management, which is where a specialist matters most.
Every UHM project runs through the same structured stages. You'll always know what's happening next, who's doing it and when it completes.
A director attends, reviews the plot and your architect's concept, and gives an honest read on planning, structural and party wall feasibility.
Engineer's underpinning and retaining wall design coordinated with your party wall surveyor and architect. Method statements produced.
Sequenced dig-out, underpinning and retaining wall installation carried out by our directly-employed crew with continuous temporary works.
Reinforced raft, Delta membrane cavity drainage, twin sump-and-pump, and light-well installation across the extended envelope.
Full conversion to your architect's specification, building control sign-off, and issue of 25-year structural and waterproofing warranties.
Basement extensions come in several distinct forms, each with its own planning, structural and waterproofing challenges. Here's the toolkit.
Excavating below and behind the existing rear wall to extend the basement under the garden. Typically finished with light wells at the garden end, delivering an open-plan lower ground floor with garden connection.
Excavation under the side return alley of a period London terrace, often combined with an above-ground side return extension. Adds a wide, well-lit lower ground floor typically used for kitchen-diners.
Many Victorian terraces have brick vaulted chambers under the front pavement. Where planning and highway consents allow, these can be brought into the waterproofed envelope, adding usable floor area at the front of the home.
For new-build homes on tight London plots, we deliver full-footprint basements below the finished house, as delivered on our Tennyson Street (Battersea) and Sutherland Grove (Southfields) projects. The engineering discipline is closest to a basement extension, integrated with the ground-up build programme.
Extending a basement isn't the same job as a standard underpin. Our 150+ basements include projects that extend under gardens, side returns and pavement vaults.
The completed extended basement carries a 25-year structural warranty and a 25-year waterproofing warranty, both documented at handover.
As Considerate Constructors members, we coordinate deliveries, muck-away and noise carefully, critical when the works cross boundary lines.
Extended basements almost always trigger party wall awards. We handle the method statements and access schedules to keep programme on track.
Because everything runs through the same directly-employed team, a complex extension programme stays coherent from start to finish.
Kitchen-diner across the whole footprint, principal suite with dressing rooms, cinema room with acoustic separation, the layouts a straight underpin can't deliver.
Extend the basement beyond the existing footprint and you introduce new party wall exposures, new drainage complications and new structural challenges. Contractors who handle a straight under-house dig can suddenly be out of their depth.
Extending under a garden brings you close to (and often under) neighbour boundaries that weren't affected by the original house. Fresh party wall notices, method statements and surveyor coordination are needed, a specialist detail.
Extended basements need retaining walls at their new edges. Cantilever design, piled retaining or hybrid solutions must be selected against the ground and neighbour profile, a competence a general builder rarely has.
The bigger the envelope, the more critical the waterproofing detail at movement joints, service penetrations and the interface between existing and new structure. Corner-cutting here shows up years later.
Choosing a contractor whose portfolio is only straight under-house digs.
Underestimating party wall implications for garden and pavement extensions.
Skipping planning pre-application advice.
Detailing waterproofing joints on site rather than at design stage.
Ignoring drainage and highway authority requirements for pavement vault works.
Treating the light-well design as an afterthought once excavation is complete.
Straight answers we give on almost every free site visit, cost, timeline, guarantees and what's included.
Almost always. Basement extensions that go beyond the existing footprint typically require full planning permission, sometimes including light-well design and pavement works. We can advise informally at feasibility stage, working alongside your architect.
Priced per project. The number reflects the scale of the extended envelope, retaining wall complexity, party wall exposure and finish level. Site visits and written quotes are free.
Larger and more complex than a straight under-house dig, extended basements typically run 12 to 18 months from start on site to handover, depending on scope. The exact programme is confirmed in writing before we start.
Yes, extended basements often involve two or three adjoining neighbours. We coordinate the technical detail each surveyor needs, and keep the programme moving as awards come through.
For most basement extensions, the upper floors of the home remain occupied through the majority of works. We're honest at feasibility if occupation genuinely won't work, normally around the interface stages.
The full 25-year structural warranty and 25-year waterproofing warranty applies to the completed extended basement, the same paperwork as any UHM basement.
Yes. Tennyson Street in Battersea and Sutherland Grove in Southfields are examples of full-basement new-build homes we've delivered.
London only, primarily South London: Wandsworth, Clapham, Wimbledon, Putney, Balham: with regular projects in Chelsea, Kensington and Richmond upon Thames.
A director will visit, review your architect's plans and site conditions, and give you a written quote and honest feasibility, for free.
