15-40 Architecture brings together the knowledge and expertise of a group of designers who have delivered over 40 Passivhaus buildings. As well as sharing an aim to radically reduce energy consumption through a fabric first approach we are also committed to making buildings from materials that have low environmental impacts and ensure healthy environments for users. Having five Passivhaus designers within the company we can offer clients an especially rich level of understanding and experience. 

Our name refers to the metric that defines the maximum space heating demand (15 to 40 kWh/m².yr) used in Passivhaus and AECB building performance standards.



Jae Cotterel

Jae is an RIBA accredited architect who also holds an MSc in Advanced Environmental & Energy Studies that led to her becoming one of the first UK Certified Passivhaus Designers. She completed one of the first three certified Passivhaus retrofit houses in the UK (a Passivhaus B&B) which won the Passivhaus Trust UK ‘“Best Private House 2013” award. She is co-author of The Passivhaus Handbook, the first comprehensive book on Passivhaus (published 2012). Jae has most recently developed a timber frame based construction solution, the PH15 System, to reliably and cost effectively deliver Passivhaus buildings. The PH15 System was shortlisted for the prestigious Ashden Awards in 2018


Andrew Goodman

Andrew is an RIBA accredited architect and has also completed Certified Passivhaus Designer and AECB CarbonLite Retrofit training. He established Good Architecture in 1990, after working for Design Group Cambridge on commercial and housing projects for Cambridge college, business and developer clients. His work focuses on low energy and environment conscious building design, in particular for self-build or community-led building projects. In 2012 he was runner-up in the UK Passivhaus Competition with a scheme that focused on low impact construction within a diverse, productive landscape. Andrew is a member of the Passivhaus Trust Technical Panel and Transition Hertford.


Harry Paticas

Harry is an architect with a passion for beautifully crafted buildings that deliver robust building performance. He is a certified Passivhaus designer with experience in new-build and deep retrofit of historic and modern buildings. Harry’s own home in south-east London is the UK’s first pre-certified step-by-step retrofit project to the Passivhaus EnerPhit standard.

Harry was named RIBA London Project Architect of the year 2018, is co-director of the RIBA Chartered Practice Arboreal Architecture and a co-founder of 15-40 Ltd. He is currently a member of the STBA (Sustainable Traditional Buildings Association) Panel of Experts and co-chair of the Technical Working Group on retrofit for the UK Centre for Moisture in Buildings.


Bob Prewett

Bob has developed a portfolio of high quality low-energy retrofits for private clients and social landlords. He is a certified Passivhaus designer and founding member of the UK Passivhaus Trust. He teaches retrofit principles on the Retrofit Academy Coordinator Course and has delivered Masterclasses for the Passivhaus Trust. He sits on the Sustainable Traditional Buildings Alliance expert panel, which led him to co-author guidance on Solid Wall Insulation for Bristol City Council. He also co-chairs the Retrofit Working Group for the UK Centre for Moisture in Buildings. He was long-listed for an Ashden award in 2019.


Tom Raymont

Tom lived in the Findhorn Foundation in north-east Scotland; a spiritual community, eco-village and international centre for holistic education before training as an architect at the Architectural Association in London. He was awarded scholarships for his work from both the Architectural Association and the RIBA.

He worked for renowned international practices Eric Owen Moss Architects (Los Angeles) and Asymptote Architecture (New York) before founding Arboreal with Harry Paticas. He is a Visiting Lecturer at the University of Westminster and has previously taught at Central Saint Martins. He is an Associate Fellow of the Higher Education Academy.

Junko Suetake

Junko qualified and practised as an architect in Japan working for several internationally acclaimed practices on public, commercial and private projects. She has extensive experience in housing, school projects, community buildings, Passivhaus private houses, retrofit projects and a large scale Passivhaus cohousing project. She was a partner at Anne Thorne Architects until 2012 where her project 1-5 Bateman Mews won an RIBA Award and was shortlisted for the 2010 Stephen Lawrence Prize. She is a Certified Passivhaus Designer since 2012 and one of the first graduates of the CarbonLite Retrofit training course.


Fran Bradshaw

Fran is an architect working on regeneration and sustainable projects, with a strong focus on place making and community involvement. She has been a partner at Anne Thorne Architects since 1996, where she led retrofit and new-build award winning schemes at Angell Town, Brixton. Fran earlier trained and worked as a bricklayer. She lectures widely on sustainability matters, and has a serious involvement in progressing environmental sustainability. Fran is ARB registered, a trustee of the AECB – the sustainable building association, and a member of the Passivhaus conference UK group. Fran was key in the formation of 15-40 Ltd.



Deep Retrofit

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Around 35% percent of UK carbon emissions are a result of operating buildings. 15-40 recognise that intervening with our existing building stock is essential if we are to meaningfully reduce carbon emissions from the sector as a whole.

Our group members have pioneered a number of deep low-energy retrofit projects that have demonstrated that substantial energy savings can be made in a responsible manner where original building fabric is enhanced or maintained. We have worked across a range of construction types and frequently within the context of conservation areas and historically listed buildings.

As well as working on built projects, we have written guidance documents on the subject such as the Passivhaus Handbook and Bristolians Guide to Solid Wall Insulation. Our team have contributed to standards such as the new PAS 2035 and provided training for the Retrofit Academy on their Retrofit Coordinator course.


Natural Materials


We pay particular attention to the materials that we source for our buildings and in doing so find that more natural materials are almost always preferable. By natural we mean materials that are mineral or plant-based and have had relatively little processing from their origin to becoming a building component. They are usually simply structured rather than a composite of multiple materials and often have a long tradition of use in construction.

The specification of a particular material is one of the key decisions in architecture and it has essential effects on how people will experience a building. Moreover, it has significant impacts – even if they are less immediately obvious – on the people and place from which the material originated and the people and place in which it may end up in the future. Materials have a life story: a past, a present and a future. We believe that it is the architect’s responsibility to guide that life story for the benefit of all.

We source materials that have been produced locally and with care for the environmental impacts of their production processes. We favour those that require less energy to make and bring to site. In construction and in use we select materials that are not toxic or irritating to work and live with. We favour those that can be installed with less wastage or where the left-overs can be easily re-used or recycled.

Wood, stone, clay and lime plaster are some of the materials that satisfy these criteria. They have excellent technical characteristics and are also a pleasure to live with, expressing tactile, sensuous qualities and aging beautifully.


Passivhaus Standards


Passivhaus is an international energy efficiency standard, the only standard proven to ensure design intent translates to real life measured performance. Energy in-use is reduced to the minimum within carefully considered economic and practical constraints - energy use could be reduced to zero, but this would not be widely economically viable. In the UK, the PH standard typically represents a 90% energy use reduction. This is achieved through high levels of insulation, reduction of air leakage through the walls/roof/floor, triple-glazed windows/doors, and a whole building ventilation solution which recovers heat from extracted air (wet, smelly air being exhausted out from bathrooms or kitchens).

The Passivhaus approach is focused on reducing our dependence on carbon emitting fuels, BUT it also enables other important outcomes. Firstly, the risk of internal condensation is removed, eliminating potential mould growth, and improving internal air quality significantly. Asthma or hay-fever sufferers will notice the difference. Secondly, the constant supply of fresh air to every habitable space, coupled with no internal draughts, ensures very high levels of thermal comfort. Thirdly, fuel poverty is eliminated - imagine no-one ever having to worry about meeting the costs of an annual heating bill, or as a social landlord being able to offer space heating within the rental charge (known as warm rent) because the cost is that minimal.

Finally, Passivhaus is a mark of quality and must be independently checked using the PHPP energy modelling software. So no greenwash, no eco babble, just a rational approach to building that is kind to our environment and to us.


Community Involvement


The key to a successful project is a design process that invites people to get involved in a way that is enjoyable and creative. We listen carefully to our clients and all project stakeholders looking for the highest potential in places and people. With larger organisations we are able to chart a course between the expression of their identities and the goals of sustainable development.

15-40 Architecture works on projects of varying sizes to provide a comprehensive service, even after your buildings are delivered. Each solution is unique and we specialise in providing a personal design service for each of our clients.

Our team work is robust because it’s rooted in a long view of community and broad experiences of designing and delivering sustainable buildings.  Our group of professionals is encouraging and innovative. We are working within an emerging culture in our industry to share knowledge, to improve living quality, and to tackle global environmental issues.