07450 310772sales@uhmbasements.co.uk FMB  ·  Delta Registered  ·  £10M Insured  ·  25-Year Warranty About Services Areas Our Work Reviews Contact 07450 310772Book a site visit Basement Extension · London # Basement extensions in London,reaching beyond your footprint,not just beneath it. 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. Book a free site visit 07450 310772 150+ basements delivered · Considerate Constructors members · Free site visit and feasibility read. 150+ Basements delivered CCS Considerate Constructors 25 yr Structural warranty £10M Insurance cover 25 Strong in-house team Recent work ## Recent projects, delivered by UHM. View the full portfolio → Service overview ## Basement extensions unlock the plan you can't build above ground. 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. What it is 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. Who it's for 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 it's needed 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. Why it matters 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. Our process ## Structured stages, no shortcuts. 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. - 01 ### Feasibility & pre-app A director attends, reviews the plot and your architect's concept, and gives an honest read on planning, structural and party wall feasibility. - 02 ### Structural & party wall design Engineer's underpinning and retaining wall design coordinated with your party wall surveyor and architect. Method statements produced. - 03 ### Extension excavation & retaining works Sequenced dig-out, underpinning and retaining wall installation carried out by our directly-employed crew with continuous temporary works. - 04 ### Raft, waterproofing & light wells Reinforced raft, Delta membrane cavity drainage, twin sump-and-pump, and light-well installation across the extended envelope. - 05 ### Fit-out, sign-off & warranty Full conversion to your architect's specification, building control sign-off, and issue of 25-year structural and waterproofing warranties. Technical detail ## Where basements can extend, and how we structure each. Basement extensions come in several distinct forms, each with its own planning, structural and waterproofing challenges. Here's the toolkit. ### Rear garden extension 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. - Cantilever or retaining wall design at new garden edge - Light wells for daylight and habitable ventilation - Waterproofing joints designed at existing/new interface - Landscaping restoration to garden above ### Side return basement 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. - Coordinated with above-ground side return - Party wall retaining wall design - Combined structural raft with above-ground extension ### Front pavement vault reinstatement 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. - Highway authority consents where needed - Structural reinforcement of vault crown - Waterproofing tied to main basement envelope ### Full-footprint new-build basements 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. Why it's worth doing properly ## Concrete outcomes, not vague claims. ### Specialists who've done it before 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. ### 25-year structural warranty The completed extended basement carries a 25-year structural warranty and a 25-year waterproofing warranty, both documented at handover. ### Neighbour management As Considerate Constructors members, we coordinate deliveries, muck-away and noise carefully, critical when the works cross boundary lines. ### Party wall discipline Extended basements almost always trigger party wall awards. We handle the method statements and access schedules to keep programme on track. ### One team, big project Because everything runs through the same directly-employed team, a complex extension programme stays coherent from start to finish. ### The plan you actually want Kitchen-diner across the whole footprint, principal suite with dressing rooms, cinema room with acoustic separation, the layouts a straight underpin can't deliver. The real cost of the wrong contractor ## A basement extension amplifies every risk of a normal basement dig. 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. ### Party wall exposure on new boundaries 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. ### Complex retaining walls 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. ### Waterproofing across a bigger envelope 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. ### Common homeowner mistakes. - 01Choosing a contractor whose portfolio is only straight under-house digs. - 02Underestimating party wall implications for garden and pavement extensions. - 03Skipping planning pre-application advice. - 04Detailing waterproofing joints on site rather than at design stage. - 05Ignoring drainage and highway authority requirements for pavement vault works. - 06Treating the light-well design as an afterthought once excavation is complete. Frequently asked ## Real answers, before you call. Straight answers we give on almost every free site visit, cost, timeline, guarantees and what's included. 07450 310772Message us ### Do basement extensions need planning permission? 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. ### How much does a basement extension cost? 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. ### How long does a basement extension take? 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. ### Do you handle multiple party wall neighbours? 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. ### Can I keep living in the house during the works? 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. ### What warranty applies to a basement extension? The full 25-year structural warranty and 25-year waterproofing warranty applies to the completed extended basement, the same paperwork as any UHM basement. ### Do you deliver full new-build basements? Yes. Tennyson Street in Battersea and Sutherland Grove in Southfields are examples of full-basement new-build homes we've delivered. ### Which parts of London do you extend basements in? London only, primarily South London: Wandsworth, Clapham, Wimbledon, Putney, Balham: with regular projects in Chelsea, Kensington and Richmond upon Thames. Related services ### Explore the full basement discipline - Basement Excavation - Basement Underpinning - Basement Conversion - Waterproofing & Tanking - Light Well Installation - View all services Where we work ### Serving South London first - London - Wandsworth - Clapham - Wimbledon - Putney - Balham - Chelsea - Kensington Book your free site visit ## Ready to extend your basement beyond the footprint? Let's read the feasibility properly. A director will visit, review your architect's plans and site conditions, and give you a written quote and honest feasibility, for free. - Free site visit - Free written quote - 25-year warranty - £10M insurance - Directly-employed teamRequest your free quote 07450 310772 Or return to the homepage · read about UHM. Underhouse Management Ltd, London's structural basement specialists. A directly-employed team of 25, delivering meticulous below-ground work across the capital's finest addresses. Established 2009. FMB·Delta Registered·Considerate Constructors ### Services - Basement Excavation - Basement Conversion - Waterproofing & Tanking - Underpinning - Cellar Conversion - Basement Extension ### Areas Served - London - Wandsworth - Clapham - Wimbledon - Putney - Balham - Chelsea - Kensington - Richmond upon Thames ### Contact - 07450 310772 - sales@uhmbasements.co.uk - Unit C220A, Trident Business Centre,89 Bickersteth Rd, London SW17 9SHBook a site visit © 2026 Underhouse Management Ltd (t/a UHM Basements). Company No. 06968521. Privacy Policy·Terms & Conditions·Cookie Policy·25-year warranty · £10M insurance Call nowBook a site visit