A development of offsite-manufactured homes in Bristol aims to demonstrate how small urban brownfield sites, such as garage plots, can become a catalyst for creating low-cost sustainable housing
The scheme of nine one-bedroom, two-storey homes was built on a council-owned disused garage yard in Bristol’s northern suburb of Horfield. Credit: BDP
With the government’s ability to deliver a targeted 1.5 million new homes in England by 2029 in question, and the impact of a swathe of new planning reforms still to be felt, the pressure is on to find speedier ways to unlock sites for development.
A resurgence in smaller-scale infill residential projects, for example on former garage or high-street gap sites in towns and cities, may be a key part of the solution. Pro-development campaign group the YIMBY Alliance has estimated that this type of opportunity could provide up to 30,000 new homes a year.
Design practice BDP devised its Gap House concept to help Bristol City Council meet an acute housing need by squeezing low-cost, factory-constructed eco-homes into small urban spaces, including garage plots.
The first scheme of nine one-bedroom, two-storey Gap homes has now been built on a council-owned disused garage yard in Bristol’s northern suburb of Horfield, close to Southmead Hospital.
BDP’s Bristol Gap House for garage sites, from inside to out. Credit: BDP
‘Huge opportunity’ in reusing garage plots for housing
Commissioned by the local authority’s housing development team and funded by its housing and landlord service, the properties were constructed, by main contractor Beard Construction and modern methods of construction (MMC) specialist Etopia, using high-performance offsite manufactured structural insulated panels (SIPs) for an estimated cost of £2,000 per m2, equivalent to around £96,000 for each home, according to BDP.
Each home has a 40m2 (8m x 5m) footprint and features an open-plan kitchen/living area and bathroom on the ground floor, and a bedroom and storage on the first floor, with a balcony area sandwiched between adjacent properties. A compact street-facing green space and outdoor seating was conceived to encourage social interaction and community connection.
High levels of insulation enabled by the use of MMC, in combination with low-energy lighting, renewable energy generated by solar PV panels and an air-source heat pump, should result in a predicted 40.5kWh/m²/yr energy use, compared with the average annual modelled energy use for homes in England in 2022 which was 231kWh/m²/yr.
Matthew Mayes, project director at BDP, tells RIBAJ: “Old garage plots represent a huge opportunity in Bristol. This type of site lends itself to repetition and replication, bringing economies of scale, which is really needed to make it work from a financial point of view.”
Each compact home features an open-plan kitchen/living area and bathroom on the ground floor, a bedroom, storage and an external balcony on the first floor. Credit: BDP
Exploiting advanced construction methods
Gap House was first unveiled by BDP at the Bristol Housing Festival in 2018 and the concept was developed further through an Innovate UK-funded demonstrator programme, initiated by Bristol City Council and a consortium of partners in 2020, to benchmark MMC home building against existing delivery models.
The council was keen to exploit advanced construction methods to address a shortage of social-rent homes by developing small brownfield sites in public ownership, located in existing communities and close to amenities.
Bristol had an estimated 2,000 garages on 300 different council housing estates suitable for redevelopment in 2018, and according to BDP’s own estimates there are 15 brownfield sites suitable to deliver 10 units each within 30 minutes’ walking radius of Southmead Hospital alone.
The homes in Horfield are a contemporary take on a terraced street, their compact form and monopitched PV-lined roofs reminiscent of Stirling Prize 2024-shortlisted Chowdhury Walk in Hackney, designed by Al-Jawad Pike – another project by a council trying to develop small, underused sites.
The SIPs forming the superstructure for the homes measure up to 6m wide by 2.4m tall. A separate glulam ring frame supports intermediate floor joists with further SIPs panels forming walls for the first floor, capped by the timber roof trusses. The windows, zinc cladding and roof joinery were installed through separate packages.
Site access to the garage plots was via a single narrow access road, making Etopia’s 2D flat-packed system preferable to bulkier volumetric modular options that require space for cranes. “Other benefits of the system include less waste, fewer people on site, which is advantageous from a health and safety point of view, and better quality,” says Mayes.
Predicted energy use is 40.5kWh/m²/year thanks to high levels of insulation, low-energy lighting, renewable energy generated by solar PV panels and an air-source heat pump. Credit: BDP
According to Reece Thorner, head of design at Etopia, the panellised MMC solution requires significant coordination, ideally starting at RIBA Stage 3, in line with guidance contained in the DfMA Overlay to the RIBA Plan of Work.
“During RIBA Stage 3 we inform the architect of specifications, buildups, and how the system interfaces with all the other systems they might not specify, including party wall details, roof details and so on,” says Thorner, adding that early engagement ensures cost transparency, with geometric design “freeze” early in RIBA Stage 4 enabling the contractor to finish its design efficiently.
The council had no BIM requirement, but the team nevertheless adopted a ‘golden thread’ approach from concept, through detailed design, manufacture of the panels, and the coordination of on site activities.
An automated plugin for Revit enabled Etopia to optimise panel sizes and arrangements to fit into structural zones within the fully coordinated BIM model, taking into account windows and door openings. This helped optimise materials cutting lists to reduce waste, also saving on design time.
“If you move a window just 50mm to the left, it could mean a lot less waste is generated, so the MMC piece is another layer we’re developing design criteria against. It’s just a tiny building, but the same logic can apply to bigger housing blocks,” says Mayes.
BDP’s design aims to be simple and robust, with a cost-effective modern palette, including a zinc wrap. Credit: Etopia
Getting over lack of test data
Working with this form of MMC raised challenges for fire compliance due to a lack of test data for specific situations, such as the party wall between properties, requiring additional tests to satisfy NHBC.
The physical constraints of the narrow garage plot also impacted the efficiency of the solar PV. “You are almost forced to adopt a certain building orientation, so the PVs are probably 20 degrees out compared with if we had a nice big greenfield site,” says Mayes.
Nevertheless, the homes are anticipated to deliver 950kWh/year renewable energy output, which in combination with an air-source heat pump should mean an overall 80 per cent reduction in annual carbon emissions. The innovative structural strategy also helped lower total whole life embodied carbon to an estimated 515kgCO₂e/m² (measured to practical completion), beating the RIBA 2030 Climate Challenge target for domestic properties of below 625kgCO₂e/m².
Construction didn’t run as smoothly as anticipated, according to Thorner. Project phasing meant Etopia’s team was frequently held up waiting for ground workers to complete, resulting in a three- to four-week delay to the programme. In pursuit of greater efficiencies in future, Etopia is planning to extend the scope of its manufacturing operation to enable prefenestrated panels and the integration of different facade treatments and M&E, translating into time savings on site.
Economies of scale should also improve if multiple developments come onstream. “If you rolled it out across five sites, then [things] would change massively,” says Mayes. “If the council partnered with Etopia for, say, a three year period, you would start to generate that cost efficiency.”
BDP is pitching Gap House schemes for two other Bristol City Council sites due to come to tender, and Mayes reports that councils in Cheltenham, Bath, and Taunton have also expressed an interest.
The houses have a glulam ring frame which supports intermediate floor joists. Credit: Etopia
The structural panels were craned into the site. Credit: Etopia
How MMC can help innovative housing schemes
Greencore Homes, based in Bicester, builds ‘climate positive’ homes made from closed timber frame panels insulated with natural materials. Ian Pritchett, the firm’s co-founder and innovation director, explains to RIBAJ the key benefits of MMC for innovative housing design.
“MMC allows for greater precision and consistency, enabling us to push sustainability boundaries while ensuring buildability and performance,” he says. “It accelerates delivery and quality, reduces waste and on-site disruption, and supports a fabric-first approach that integrates design and performance from the outset.”
For Greencore, it’s also a pathway to sustainability. “Our MMC system uses low-carbon materials like hemp, lime, and wood fibre, allowing us to design and deliver homes with a structure that is better than net zero embodied carbon and excellent thermal performance,” Pritchett adds.
“It’s vital architects work closely with us from the outset to align the design with the structural parameters and environmental performance standards of our panel system,” he says. “Understanding our system’s capabilities, such as allowable spans, window detailing, and insulation strategy, ensures smoother progression from concept to manufacture.”