Passivhaus Affiliate

First UK retrofit certified to Passivhaus Plus

This Bedfordshire renovation surpassed EnerPHit criteria to become the UK's first certified Passivhaus Plus retrofit!

Greenfield Passivhaus Plus. © AD Practice, Matthew Smith Architectural Photography

Completed in 2023, Greenfield Passivhaus Plus is a holistic retrofit of a 1960s detached home in Bedford. The design, by PHT member A D practice, entailed reconfiguring the internal space, adding a mezzanine and a new roof. The clients desired a new lease of life for their home whilst reducing their environmental impact and creating a dwelling capable of adapting with them through life.

Interiors have been completely modernised and flooded with daylight.

Key stats

  • Construction: Hybrid

  • TFA: 192 m2 

  • Build start date: 2022

  • Completed: 2023

  • Certified: Passivhaus Plus 

Greenfield Passivhaus Plus © AD Practice, Matthew Smith Architectural Photography



The existing ground floor was rreplaced with a new highly insulated slab with underfloor heating and cooling. The existing brick cavity walls were externally insulated using wood fibreboard that was finished with a painted render and vertical Brimstone Ash cladding. The new first-floor gable walls are highly insulated timber I-beams with cellulose insulation and clad in vertical Brimstone Ash cladding to match the upgraded brickwork. The existing roof was removed and turned 90 degrees. The new roof was constructed similarly to the new walls, forming a gable at the front and rear, maximising the views across the garden and countryside beyond.


Floor: 0.127W/m2K

Exterior - interior: Concrete slab, 160mm PIR insulation, Concrete screed with under floor heating, Floor finish.

Wall: 0.126W/m2K

Retrofitted Wall:

200mm woodfibre EWI, Existing insulated brick and block cavity wall, Airtightness membrane, Service Void, Plasterboard and skim.

New First Floor Wall:

200mm woodfibre insulation, OSB, 200mm woodfibre insulation in between timber studs, SmartPly, Service void, Plasterboard and skim.

Roof: 0.117W/m2K

60mm rigid woodfribre insulation, 300mm blown cellulose insulation in between timber I joists, Smartply
Service void, Plasterboard and skim.

Further information can be found at the International Passivhaus Database listing


Greenfield Passivhaus Plus_Kitchen after. ©  AD Practice, Matthew Smith Architectural Photography Greenfield Passivhaus Plus_bedroom after. ©  AD Practice, Matthew Smith Architectural Photography Greenfield Passivhaus Plus_Living room after. ©  AD Practice, Matthew Smith Architectural Photography
Greenfield Passivhaus Plus_Kitchen before. ©  AD Practice Greenfield Passivhaus Plus_bedroom before. ©  AD Practice Greenfield Passivhaus Plus_living room before. ©  AD Practice
Before and after interiors. Top row after - kitchen, bedroom, living room. Bottom row before.

Building performance

Designed energy performance, calculated according to PHPP

Airtightness n50 (≤ 0.6ACH @ 50 Pa)                           


0.6 @ 50 Pa

Space Heating Demand** (≤ 15 kWh/m².a)


18 kWh/m².a  

Heating Load** (≤ 10 W/m²)


10 W/m²

Primary Energy Demand** (≤ 120 kWh/m².a)



PER demand 30

Primary Energy Renewable (PER) Demand** (≤ 60 kWh/m².a*)    


71 kWh/m².a

Primary Energy Renewable Generation


79 kWh/m².a 

*+/-15 kWh/m².a allowance if offset by energy generation. See Passivhaus criteria
**Space Heating Demand, Heating Load, Primary Energy Demand, Primary Energy Renewable Demand - All currently undergoing monitoring.

Greenfield Before - Back Exterior ©  AD Practice
Greenfield After - Back Exterior ©  AD Practice, Matthew Smith Architectural Photography
Before and after (back exterior)


The client wanted a 'gas-free' house. The home does not use fossil fuels. A ground source heat pump and an entire edge-to-edge roof pitch dedicated to 12.5kW solar PV help meet much of the home's energy requirements.

Greenfield Solar PV © GB-Sol Limited

Lessons learned 

  • Working with roof-integrated solar PV was a huge success. 

  • Building & reorientating a new roof eliminated thermal bridges and allowed a full roof slope of solar PV to be achieved.

  • Making old cavity walls airtight where they had become internal through a previous extension and acted as a chimney result in meeting 0.6ach rather than the 1.0 for EnerPHit.


Greenfield Before - Front of House © AD Practice Greenfield After - Front of House ©  AD Practice, Matthew Smith Architectural Photography
Before and after (front exterior)


Key team 

  • Client: Private

  • Architect & Passivhaus Designer: PHT member A D Practice Ltd

  • Contractor: Apollo Construction Projects Ltd

  • Structural engineer: Rawlings Structural Design Ltd

  • Passivhaus Certifier: PHT member Mead Consulting

Greenfield Passivhaus plus © AD Practice, Matthew Smith Architectural Photography

Achieving the Passivhaus Plus standard for a retrofit is a huge accomplishment! Congratulations to the whole team & AD Practice, who have fully embraced the Passivhaus retrofit. It is not the architect's first -  Harpenden EnerPHit Plus was a finalist in the 2023 UK Passivhaus Awards. We eagerly await their next Passivhaus project!


Architects view

We are always very keen to support clients who see the environmental benefits of upgrading existing properties to EnerPHit standard or, as in this case, beyond, rather than the often cheaper but less sustainable option of a demolition and rebuild. Our clients are delighted with the end result, and to have the exclusive tag of living in the first Passivhaus Plus retrofit in the UK!” 

Heather McNeillDirector, Architect & Certified Passivhaus DesignerA D Practice Ltd


Similar projects 

The projects below are all EnerPHit Plus certified. Take a look at our Plus & Premium page and Retrofit page for further examples, and to get started on your Passivhus self-build journey, visit the Passivhaus goes Personal page. For more guidance, check out the How to Build a Passivhaus: Good Practice Guide or our FREE interactive Introduction to Passivhaus online course.  

Harpenden EnerPHit Plus
  Shepherds Barn EnerPHit Plus
  Modernist EnerPHit Plus


Further information 

Greenfield Passivhaus Plus

Passivhaus Retrofit

Passivhaus goes Personal

Research Report - Passivhaus Retrofit in the UK

Technical Guidance - Moisture and EnerPHit

Passivhaus Benefits Guide & costs research 

Passivhaus Powerhouses (Plus and Premium)

Passivhaus on-demand training 

Before and After (Back Exterior)

12th February 2024

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