From Victorian villa to EnerPHit exemplar: Hampstead's heritage retrofit

A Victorian semi-detached villa near Hampstead Heath has been transformed into a compelling deep retrofit projects in London - achieving certified EnerPHit standard within a Conservation Area, and shortlisted for the RIBA London Awards 2026.

As Guy Martin's House Without Bills brought Passivhaus retrofit into living rooms across the UK earlier this year, this certified EnerPHit in North London, demonstrates how the standard can be applied to one of Britain's most challenging building types: the Victorian terraced villa.

This project transformed a semi-detached Victorian villa near Hampstead Heath into a high-performance, future-ready family home, demonstrating how historic buildings can be sensitively upgraded to meet today's environmental demands. Delivered within a Conservation Area, the renovation and extension involved a deep energy retrofit to achieve the rigorous EnerPHit standard - an internationally recognised benchmark for major refurbishment.

Key stats

Standard: Passivhaus EnerPHit
Construction type: Masonry (existing), timber frame (new extension)
Treated Floor Area (TFA): 287 m²
Completed & certified: 2023

Hampstead Passivhaus. Image Credit: Matt Clayton/ Guy Stansfeld Architecture

This ambition led us to target the rigorous Passivhaus retrofit target, EnerPHit. A benchmark we're proud to say we achieved, delivering an exceptionally high-quality, low-energy home perfectly suited to contemporary living.

Rachel Miller, Partner, Guy Stansfeld Architects

Construction

A fabric-first approach underpinned every decision on site. The existing solid masonry envelope was comprehensively upgraded using wood fibre insulation and lime-based internal linings, maintaining vapour permeability and supporting the house's original vapour open construction. Moisture management is a critical consideration for any Passivhaus retrofit of historic fabric. Airtightness, thermal continuity and moisture management were carefully detailed throughout, and triple-glazed timber windows were designed to preserve the historic proportions of the Victorian elevations while significantly reducing heat loss.

An existing rear conservatory and ancillary extensions were replaced with a new timber-framed addition, expanding the main family living space. The extension is visually and materially compatible with the historic house. Reclaimed brick and natural materials allow it to sit comfortably within its Conservation Area context. Its scale, massing and detailing clearly expressing new work while respecting the old.

Hampstead Passivhaus. Image Credit: Matt Clayton/ Guy Stansfeld Architecture

Delivering EnerPHit within a historic, occupied urban context required rigorous coordination, specialist oversight and careful budget management across all trades. Airtightness, ventilation and thermal detailing were achieved without compromising heritage values. A balance the team credit to early and sustained collaboration between architect, building physics consultant and contractor.

"We achieved Passivhaus airtightness with the application of traditional lime plaster - ideal as it is vapour open," explains Rachel Miller. "Applied with gentle movement and without plaster beads to avoid crisp corners, the walls and ceilings have a subtle softness, providing the perfect backdrop."

U-values
External wall - 0.226 W/(m²K) Internal insulation to existing solid brick; wood fibre with lime-based lining	Basement / ground floor slab - 0.129 W/(m²K) Suspended insulated floor	Roof - 0.164 W/(m²K)
Windows (frame) - Uw 0.85 W/(m²K)	Glazing - Ug 0.5 W/(m²K), g-value 54%	Entrance door - Ud 0.8 W/(m²K)

U-values

External wall - 0.226 W/(m²K)

Internal insulation to existing solid brick; wood fibre with lime-based lining

Basement / ground floor slab - 0.129 W/(m²K)

Suspended insulated floor

Roof - 0.164 W/(m²K)

Windows (frame) - Uw 0.85 W/(m²K)

Glazing - Ug 0.5 W/(m²K), g-value 54%

Entrance door - Ud 0.8 W/(m²K)

Embodied carbon

Hampstead Passivhaus. Image Credit: Matt Clayton/ Guy Stansfeld Architecture Where possible, the existing building fabric was preserved. The original staircase, structural elements and many floorboards were retained - preserving character and avoiding unnecessary material loss. New materials and finishes were carefully selected to minimise environmental impact, avoiding petrochemical-based products and incorporating reclaimed or repurposed elements wherever possible.

Using natural materials that were either reclaimed, some from the site itself, or responsibly sourced resulted in an authentic and tactile quality that is apparent throughout the house. Kitchen worktops were cut from slate reclaimed from snooker tables, complete with original brass studs and pocket cut-outs, filled with wood discs, brass caps and an eco-based resin. Cupboard fronts were made from boards salvaged from cheese production. Floorboards were reused as soffit cladding. Even clay soil excavated from the rear garden was mixed into hand-crafted paint to create a unique, site-specific colour palette.

The structural choice for the new rear extension further reflects this ethos: timber was selected over steel for its substantially lower embodied carbon. Concrete was avoided wherever possible. The team acknowledge that any modernisation increases embodied carbon in the short term, but that the whole-life carbon payback of a Passivhaus upgrade is demonstrably faster than a standard refurbishment.

Building performance

Measured and modelled energy performance
Airtightness n50 (EnerPHit limit ≤ 1.0 ACH @ 50 Pa)	0.85 ACH @ 50 Pa
Annual space heating demand	20 kWh/(m²a) via PHPP
Heating load	12 W/m²
Primary Energy Renewable (PER) demand Heating, DHW, household & auxiliary electricity	57 kWh/(m²a) via PHPP
On-site renewable energy generation	34 kWh/(m²a) based on projected area

The airtightness result of 0.85 ACH is a notable achievement for a solid masonry Victorian property undergoing retrofit within an occupied Conservation Area context - well within the EnerPHit limit of 1.0 ACH and requiring meticulous detailing of junctions, penetrations and party wall interfaces. The annual heating demand of 20 kWh/(m²a) represents a dramatic reduction against the unimproved baseline, and the house now operates with no fossil fuels on site. Further technical details can be found on the International Passive House Buildings Database: Entry #7235

Services

Building services were completely overhauled to support low-energy living. A Zehnder Comfoair Q600 mechanical ventilation with heat recovery (MVHR) unit with integral pre-heater for frost protection ensures a continuous supply of fresh, filtered air throughout the home while minimising heat loss. This not only reduces energy demand but delivers the consistent indoor air quality and stable temperatures that are a hallmark of certified Passivhaus performance.

A 6 kW rooftop photovoltaic array contributes significantly towards the home's minimal energy demands, generating 34 kWh/(m²a). Domestic hot water is provided by a Mixergy hot water storage cylinder. Low-energy lighting and water-efficient fittings further reduce operational impact. The house is fully electric, no gas connection or fossil fuels, made viable by the dramatically reduced heating demand achieved through the building fabric.

Hampstead Passivhaus. Image Credit: Guy Stansfeld Architecture

Summer comfort was considered throughout. The MVHR's summer bypass capability, combined with openable windows and carefully managed solar access, addresses overheating risk, an essential consideration for any south-facing London property and an area of growing focus in Passivhaus design and certification.

This is one of those projects where everyone, from the clients, architects and building contractor to the interior designer - put so much thought into every detail. Everyone on this project cared about environmental sustainability from the start and kept that focus at the core of the whole process.

Ecospheric, Building Physics and Services Consultant

Heritage, conservation and replication

Planning constraints governed window proportions, external materials and the scale of the rear extension. Triple-glazed sash windows, specified with a U value of 0.85 W/(m²K), required careful negotiation to demonstrate that high performance and period-appropriate appearance are not mutually exclusive.

The UK's existing Victorian housing stock is one of the most common domestic building type in England. The challenges here (solid masonry walls, suspended timber floors, sash windows, complex party wall interfaces, occupied context) are the very challenges that face millions of comparable homes. The lessons learned at Hampstead have broad relevance.

We went to great lengths to retain the original staircase, insulating behind it and re-tying it in structurally. It felt important to retain and preserve such a significant original feature that is the core through the house and marked by the passage of time.

Rachel Miller, Partner, Guy Stansfeld Architects

Lessons learned

Integrate early and stay integrated: The involvement of building physics consultants Ecospheric from the outset, covering both PHPP modelling and M&E design, enabled coordinated decision-making and avoided late-stage conflicts between fabric and services strategy.
Breathability and airtightness are not in conflict: Wood fibre insulation and lime-based linings provided the vapour-open assembly needed for the historic masonry, while still meeting EnerPHit airtightness requirements. The two goals must be designed together, not treated as opposites.
Material strategy is part of the sustainability story: Minimising embodied carbon through material retention, reuse and responsible sourcing is as important as operational energy in whole-life carbon terms. Document and celebrate these decisions - they matter to clients, planners and future owners.
Conservation Area constraints are navigable: High-performance glazing, lime finishes and a contextually appropriate timber extension demonstrate that heritage protection and deep retrofit are compatible. Early dialogue with the Local Planning Authority is essential.
Client ambition is transformative: The clients' stated desire to demonstrate what is possible, not just to improve their own home, shaped every decision on this project. That ethos of demonstration is what the Passivhaus Trust exists to support and amplify.

Key team

Client: Private homeowner
Architect: Guy Stansfeld Architects with 318 Studio
Passivhaus Certifier: Zero Energy Passivhaus

Contractor: OLF Construction

Passivhaus Consultant: Ecospheric
Building Services: Ecospheric; Calibre Climate
Structural Engineer: MBOK
Interior Design: Axel Vervoordt

This project demonstrates that certified EnerPHit retrofit of Victorian housing within a London Conservation Area is not only achievable but capable of producing results that are architecturally distinguished, materially rich and technically exemplary. Its RIBA London Awards 2026 shortlisting is recognition well deserved, and a signal that the design community is increasingly prepared to celebrate the full ambition of deep retrofit.

As the UK Government advances the Future Homes Standard and retrofit targets grow more urgent, projects like this one provide both proof of concept and a compelling invitation: the Victorian terrace that defines so much of Britain's built environment is ready to become one of its most efficient.

Unless otherwise stated all image credits: Matt Clayton Photography / GS Architecture.