The ideas behind Alma-nac’s House Within a House stem from a line of investigation begun by its infill Slim House of 2012 and developed in its In-Betweeny House (2017) and Wedge House (2018). The premise is that small housing projects on awkward infill sites can seed big ideas, introducing experimental ways of using space and therefore new typologies that will eventually be explored at larger scale – as demonstrated in the practice’s recent retrofit of Paxton House in Croydon to create 43 flats. Domestic infill projects, the argument goes, offer a better chance for architectural experimentation simply because – in contrast to the risk-averse development sector – money-making is not always the opening objective.
This project doesn’t so much infill as overwrap what was a highly incongruous late-1950s two-storey box – one of a spattering of pockmarks in the south-east London suburb of Brockley, testament to the flight paths of Luftwaffe bombers and V1 and V2 missiles heading for London’s docks during the Second World War.
Loomed-over by a bereft twin in high-Victorian eclectic style the utilitarian brick cube unsettled the street’s stately rhythm of paired villas and terraces but at the same time attested to the veering socio-economic fortunes of a London neighbourhood conjured speculatively (and optimistically) from pattern books in the late 19th-century building boom.
Clients Michelle Anderson and Dan Witchell bought the house in 2015, finding that, despite its big plot, the small house in a conservation area was of little interest to developers because of a single-dwelling covenant in the deeds. Sparked to build by visiting Open House projects, they were looking to create a six-bedroom house in their existing locality to accommodate their family of five boys.
The big conversation about whether to replace or extend the three-bedroom house was steered largely by economics. Balancing a finite budget funded by a self-build mortgage, the clients knew that a tabula rasa approach would be zero-rated for VAT, whereas renovation and extension, they assumed, would be subject to 20 per cent. They discovered, however, that because the house had been unoccupied for two years, this liability would be reduced to 5 per cent, and furthermore that the demolition alone would cost about £25,000.
Alma-nac director Tristan Wigfall explains that the practice’s proposal was essentially to cloak the existing house – a thermally leaky solid brickwork box with no insulation to speak of – with a strategic series of ambitious extensions. ‘In effect we wrapped the entire structure in a warm coat,’ he says.
The design sequence of semi-autonomous moves added a side-slice of new build, a pavilion of back extension and a lightweight timber upper storey – all united by the envelope of a new brick skin to create super-insulated 450–550mm cavity walls in conjunction with the retained brick shell. The existing trench foundations were infilled with concrete, creating a thermal sink but also acting as part of a structural strategy that loaded the new brick skin off the original footings. Overhead, a warm-roof approach to insulation ensured that heat loss was further minimised. ‘Even timber conducts to some extent,’ Wigfall points out. He has calculated that retaining the shell and foundations prevented more than 12,500 bricks and 12.8m3 of concrete from ending up in landfill.
The juggling act of saving embodied carbon via reuse; of laying a new concrete floor but offsetting this by energy savings in use, seems to have paid off. A post-completion energy analysis has shown that the estimated annual CO2 emissions of 1.6kg/m2 have been reduced to 16.5kg/m2 (which also moves it from Energy Performance Certificate E to B).
The extended house almost doubles the floor area to 233m2, providing three extra bedrooms and versatile ground-floor living space, adaptable to the needs of a large family with children ranging in age from five to 17. Existing internal partition walls were largely retained, with sliding room dividers replacing some of the doors, so that the kitchen back extension and front-of-house study and TV room, with their deeply punched reveals, now radiate around a snug-like inner chamber of living space. Externally, the window openings, which broadly reflect the positioning of apertures within the original shell, are more delicately accentuated by steel cuffs.
The top-lit single-storey side extension plays a key role in bringing the front door back to the street (the house was previously accessed from a side passage). This new entrance is emphasised in the external plane by a switch from stretcher-bond brickwork to soldier course. Internally, it introduces a ceremonious entrance sequence which links up intelligently with the unchanged positioning of vertical circulation, now illuminated by rooflights set into the new vaulted roof.
The volumetric gains of the new house equal those of its floor space, with the added upper floor open to the rafters beneath a ridge that extends 4.5m to match the gable of the neighbouring house. The roofs of the one-storey garden pavilion and the house itself are constructed from Kerto beams and soffits on a flexible grid-based on 400mm centres. The method is similar in principle to glulam or CLT construction but here assembled on site from pre-cut components.
The interior language is pragmatic: joist hangers and bolts in the new vaulted roofs are exposed; the paint finish is white; and power sockets and some of the ceiling lights are robustly non-recessed. Most of the materials might be found on the shelves of a local builders’ merchant.
Back on the street, the whiteness of the engobed (dip-coated) brick choice is an unexpected but smart move. It has the effect of recessing the new volume against the age-darkened London stock of its neighbours while alluding to their yellower-hued Victorian beginnings.
In contrast with the exuberant, undulating depth of its neighbours, the sleek envelope of the newcomer presents a ghosted outline that implies history without resorting to a comfort blanket of pastiche. This is a long-life, loose-fit, low-energy approach to building that extends a pragmatic tradition of adapting existing fabric – think of the 18th-century craze for ‘formalising’ the façades of older buildings. But here the façade is intelligent; encompassing evolved spatial organisation and volumetric gain, as well as new-found levels of thermal efficiency. It is interesting to see that the reality of a budget can be a factor driving environmental decisions. Less is sometimes more.
Engineer’s view
The project’s structural challenges mostly concerned retaining as much of the existing two-storey structure as possible while ensuring that the finished project still looked like a single, deliberate construction.
The existing building was a typical post-war construction with suspended timber floors and roofs spanning between loadbearing masonry walls. While steel beams and posts were used to replace loadbearing walls within the existing building footprint, the new second floor was designed as a timber-framed element, using Kerto LVL S-beams where the rafters were to be exposed. The combination of a timber frame and the replacement of the suspended ground floor with a ground-bearing slab helped offset additional loads caused by the extra storey and minimised total loads. As a result, we were able to justify reusing the existing concrete foundations.
To achieve the image of a single construction, both the existing masonry walls and the new timber frame are clad in a brick skin. This is restrained by the existing masonry structure using retrofit ties while timber-frame ties are used from second floor level and above. A series of galvanised, external steel angles around the building perimeter allow for details like the toothed step in the façade, the cladding support over the side extension and the brick cladding that appears to extend through the rear extension roof structure and into the space.
Brian Constant, director, Constant Structural Design
Working detail
A key strategy in visually transforming the existing structure was the over-cladding of the façades with a homogeneous brick skin. The basic principle of tying new masonry to old with remedial wall ties is not, in itself, groundbreaking. However, in this instance we also had to take account of the timber-framed structure to be added on top of the existing building, which needed a different set of ties and, more critically, moved in a different way to the existing solid masonry walls. The specification of the wall ties needed to accommodate the relative differential movement rates between the two elements without cracking the external brick.
A further consideration was that a section of the front façade was to project half a brick out in front of the main façade to provide relief. The larger cavity behind the projecting section required a third type of tie design. Expansion joints were also needed to allow this front section to expand and contract independently of the main façade. These were concealed in the corners but maintained a longitudinal tie so that the localised thick section of brickwork formed by the step in plan could be used as a mini brick pier to help restrain the façade and stabilise the brickwork.
These interfaces, including the area where there were two different base structures, two different cavity widths and a movement joint coinciding, were challenging to visualise but the whole building was modelled in BIM both architecturally and structurally, which meant the interfaces could be co-ordinated ahead of any site work.
Architecturally we were adamant the brick should not seek to create a pastiche of the neighbouring historic buildings. We selected a warm grey brick that complements the neighbourhood’s buff London stock brick, without trying to mimic the naturally aged material. The flush lime mortar was carefully matched to the bricks to create a more uniform texture across the surface while their engobe finish adds subtle variation in tone.
Tristan Wigfall, director, Alma-nac, and Brian Constant, director, Constant Structural Design
Project data
Start on site July 2017
Completion July 2018
Gross internal floor area 233m2
Constructioncost Undisclosed
Architect Alma-nac
Clients Dan Witchell and Michelle Anderson
Structural engineer Constant Structural Design
CDM co-ordinator Alma‑nac (pre-construction), David Stewart Building Contractor (construction phase)
Approved building inspector Jhai
Main contractor David Stewart Building Contractor
CAD software used ArchiCAD
Energy Assessor Vision Energy
Performance data
On-site energy generation None
Total energy load 105.27 kWh/m2/yr (previously 350 kWh/m2/yr)
Annual CO2 emissions 4,425kg (previously 7,579kg)
Annual CO2 emissions per m2 16.49 kg (est)
Total energy cost over 15 years £14,329 (previously £25,000)
U-values (W/m2 K)
Ground floor (insulated concrete slab): 0.15
Timber external walls (ground, rear): 0.13
Brick/block external walls: 0.15
Timber/masonry external walls: 0.13
Rear extension roof: 0.14
Main roof: 0.13
EPC rating C78 (existing E40)
EI rating C73 (existing F35)
