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UK's First Ever Certified Passivhaus Plus Retrofit

We are extremely proud to announce that our Bedfordshire Retrofit project has become the first ever certified Passivhaus Plus Retrofit in the UK!


"We engaged A D Practice for the renovation of our 1970’s typical estate house to a passive-standard eco house. They gave us initial advice, developed the architectural design for planning permission, and oversaw the project throughout the build. We love our ‘new’ house, and we very much enjoyed the collaborative approach and the friendly support we have had from A D Practice throughout the project."

Nick and Robin Tillett, Greenfield, Bedfordshire


This project was unusual from the start as the clients approached us at the height of the first lockdown during the Covid-19 pandemic (April 2020) so we were unable to visit the site for quite some time and all design discussions were carried out online. This meant the initial site visit became a whole office meeting on site when we could finally go in June 2020!

Due to the lockdown the clients carried out a full measured survey which we drew up and prepared a scheme, all via the joys of video calls! The main driver was the energy efficiency retrofit of the property but the clients also wanted to improve the first floor bedrooms at the back which were only served by rooflights, completely ignoring the beautiful views over the fields that the property is blessed with. So, along with some minor internal changes to create a more open-plan living space on the ground floor, the main biggest alteration involved removing the roof and 'turning' the pitch through 90 degrees. This formed a gable to the rear meaning the bedrooms on the first floor, and newly formed mezzanine in the loft space, have beautiful views over the garden and fields beyond. It also had the added benefit of providing a south-facing roof slope which was entirely utilised for pv solar generation.


Planning was submitted in August 2020 and was a non-contentious process. Following this, the clients looked at various suppliers for windows and doors and visited our Harpenden EnerPHit Plus project to get a feel for the build process. Building regulations were applied for and the PHPP and associated details completed to gain pre-construction certification. The clients also undertook their own project of designing a ground source heat pump system that would provide both heating and cooling to the new floor slab in the house!


Several contractors tendered for the project and Apollo were appointed in November 2021 and the contract was signed in April 2022 due to the delayed start date and adjustments to material prices.

The build started in earnest in May 2022. The building was stripped, the existing concrete floor slab removed and replaced with an insulated concrete slab. The perimeter of the building was excavated to the top of the foundation level and the walls were insulated below ground with a closed cell rigid insulation. The existing brickwork walls were externally insulated with woodfibre board above DPC level. The roof was removed and the walls at first floor level at the front and rear were built up using an woodfibre-insulated timber twin stud construction to form the new gable ends before a new roof structure was formed using timber I joists with Warmcel insulation in between.


The existing walls were lined out with an Intello Plus airtightness membrane which was taped to the Smartply lining the new timber framed walls and roof. Windows were also installed into Smartply boxes taped to the internal airtightness layer and suspended into the woodfibre insulation externally to ensure thermal continuity.

The house was finished with a breathable silicone render on the ground floor and Brimstone Ash timber cladding to the first floor. Windows and MVHR were sourced from Green Building Store and the GSHP was supplied by Kensa and designed by our clients in conjunction with John Cantor. The seamless solar pv array on the south-facing roof slope was designed, supplied and installed by GB-Sol and the Passivhaus Certifier is Kym Mead.


The house is bright, airy, comfortable, and highly functional for our clients' way of living and they are thrilled with the result.


We thoroughly enjoyed working with the entire team on the project and are stoked to see the end result meet the new build Passivhaus standards.



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