Alma-nac’s house hides another house inside

At one point, architect alma-nac was thinking of naming its latest residential project the Russian Doll House before settling for the equally appropriate House-within-a-house. Both are a reference to something you’d never guess from walking past this contemporary addition to a Victorian terraced street in Brockley, south east London – that, hidden inside, is the structure of a much smaller, 1950s house.

It’s a fascinating story of how extreme retrofit can deliver a house that is not only substantially bigger, but also far more thermally efficient, while avoiding a pastiche response to the leafy conservation area setting.

The clients, a couple working in the creative industries, were looking for a site with the potential to extend to accommodate their five sons. They struck gold, as it turned out, when they spotted a sorry-looking, three bed detached house in a road of far larger properties. The house was built on a bomb site of what had been a three storey, Victorian semi-detached home, and rising to just two storeys, it was somewhat marooned on its generous site.

After initially considering a modest extension, the clients enlisted alma-nac to create a more ambitious project. According to director Tristan Wigfall, although a rather simpler demolition and new build approach was considered, they went instead for a retrofit, because the original masonry structure was sound and capable of supporting an additional storey. This more sustainable option retained the structure but extended upwards and at the rear and side, while wrapping the whole lot in a thermally-efficient cloak with a new brick skin. The resulting six-bed house is 380m² (GIA) compared to the 233m² original.

‘Our gut reaction was why knock it down?’ said Wigfall, adding that although the form was wrong, the footprint was in the right place. He also wondered whether the planners had given the architect more licence in the composition because it was re-using the 1950s structure. 

And financially, the clients were able to save not only the demolition cost but also benefit from VAT of only 5% because the property had been vacant for more than two years. The result was a re-use of 12,670 bricks and 12.85m³ concrete.

In the composition, alma-nac sought to respond to the character of the conservation area, where London stock brick was prevalent on houses of mostly three storeys, many with steeply pitched roofs and considerable facade articulation in the form of bays and decorative detailing. The three-storey design mediates the storey gap in height between its two neighbours with the inclusion of a pitched main roof that extends higher than it needs to in order to match the height of the taller adjacent house, plus a subsidiary pitched bay that relates to the height of the shorter house on the other side. 

 Rather than attempting to match the brick of the surrounding houses using recycled London stock brick, alma-nac chose a contrasting pale brick by Belgium manufacturer Floren. The relatively austere composition, which meets the clients’ preference for a pared-back utilitarian aesthetic, is animated by a small area of soldier course brick adjacent to the side-access entrance, and by projecting, powder-coated aluminium ‘cuffs’ on some of the casement windows at the front. These windows follow the general pattern of the 1950s house on the ground and first floor, with a large picture window on the site of the former garage. As a result of the new skin, these have deep reveals of up to 600mm on the projecting bay.

The new upper storey is timber-framed to minimise the load on the original structure. Internally there are just a few traces of the original house – the former perimeter wall is visible in the entrance lobby and the original stair position has been retained, along with the general arrangement of the first floor bedrooms. Otherwise, the ground floor has been reconfigured to give a more open, expansive design, with the kitchen/dining room at the rear in an exposed timber-framed extension. This was created using Kerto large-span Metsä Wood timbers with an asymmetric pitched roof form, and is clad in stained Western red cedar from Vastern Timber. The main clerestory-lit bedroom plus two others are on the top floor, the lofty rooms rising to 4.5m in height.

To achieve the new thermally-efficient ‘cloak’, alma-nac first added a 150mm concrete floor slab and the timber upper and rear extensions before building the new envelope. The slab is topped with 100mm Kingspan Thermafloor TR70 rigid insulation, underfloor heating and 100mm thick floor screed.

For the elevation, the new outer skin is Floren’s Polaris brick, laid with a lime-mix mortar that tones with the pinky-grey variegated hues. Between the brick and the existing masonry on the lower two levels is a 25mm cavity (rising to 125mm on the bay) and 50mm of Kingspan Kooltherm K108 cavity board, which is fixed to the outside of the original 215mm brick wall. A further 57.5mm of Kooltherm K118 insulation is fixed to the inside of the ­masonry wall.  

On the top floor extension, the elevation build up consists of the same brick and cavity over a timber framed external wall. This is insulated with 150mm of Kingspan Kooltherm K112 framing board between the studs. On each level, the brick is tied back to the main structure using special remedial brick ties by Ancon that allow for the variation in movement ­bet­ween the masonry and timber structures. 

Where the house is extended at the rear, a steel across the back supports the new brick facade, with more steel along the side, where the house is extended by 1m to take in what was originally a side passage to the garden. The main pitched roof is a warm roof structure with rigid insulation (Kingspan Kooltherm K107) and topped by natural slate. The rear extension has a pre-weathered zinc roof.

The thermal performance vastly exceeds Building Regulations, and gives a comparative reduction in annual CO₂ emissions from 61.55kgCO₂/m² to 16.49kgCO₂/m². U values are 0.15 W/m²K for both the brick/block external walls on the ground/first floor and for the insulated ground bearing concrete slab, and 0.13 W/m²K for the timber external walls at ground level at the rear, the timber/masonry external walls at second floor level, and the main roof. The rear extension roof is 0.14 W/m²K.

After one year on site and another finishing the interior, the family is enjoying living in the house-within-a-house, and appreciating that aside from the underfloor heating, there is rarely a need to use the radiators, even in winter. 

For the architect, it’s also a positive outcome that fits into the wider conversation about the need to consider retrofitting by making creative use of existing structures.

‘We could have made our life much simpler by demolishing and starting afresh,’ says Wigfall.  ‘However, we think taking on this challenge has resulted in a much richer project that meets the clients’ brief and provides an efficient contemporary home for a large family.’