Three outstanding extensions to Grade II-listed houses provide design inspiration and practical insights for architects looking to extend heritage buildings
How should you approach designing an extension to a listed building? The process is naturally more complex, and restricted by regulation, than altering unprotected structures.
What do discussions with a heritage consultant look like? How do you make your case in the Heritage Impact Assessment (HIA) and Design and Access Statement (DAS)? In what ways can you mitigate harm while delivering an ambitious new space – and demonstrate that intent and capability during the planning process?
Here, the architects of a bold corten-clad side extension to a Victorian Arts and Crafts home, the sensitive expansion of a Hertfordshire farmhouse and a crisp modern addition to a Georgian property offer design ideas, advice and lay bare the process.
Sibhaven, North West London, by Sanya Polescuk Architects
What is the existing house, and how have you extended it?
Sibhaven is a Grade ll-listed, three-storey detached house in Highgate designed by Michael Bunney in 1899. This was prior to his joining Clifford Copeman Mankins, a key practice in the design of much of the Hampstead Garden Suburb.
The extension replaces the ‘garage for an automobile’ built in 1955, which had replaced the original rear entrance and an outside toilet. At no more than 2m wide, the garage could accommodate few 21st-century vehicles and had gradually morphed into storage-cum-utility space. This created a perfect opportunity for a purpose-designed new facility.
How did you reach a decision about appropriate size and architectural character?
We had been working on phased alterations of the house for years, aiming to transform its multiple late-Victorian service and live-in-servants' rooms into liveable spaces. The new services, the plant, utility, and muddy-boots entrance had to go into the side extension.
Given the narrowness of the side slot, we maximised the length and height. Set back from the front-facing facade, it clears the side windows and crouches just below the prominent dentil mouldings. However, this subordinate position is offset by its confident Corten cladding, which draws on the reddish hue of the main house.
What assessment did you make of the building's special interest, and how did you conserve it?
We analysed its history, original design and alterations. This illustrated how socio-economic impetuses, typically impinging on domestic architecture, shaped the ordering and re-orderings of its internal space. Alterations were recurrent throughout its history but were focused principally in service areas. That process did not strip the building of its special interest, as proved by its listing in 2000. Our alterations continued the same logic, conserving and reusing the interior features that so clearly underpinned the hierarchy of the spatial design.
During the design process, did you work with a heritage consultant or conduct your own research?
We enjoyed doing our own historical research. It informed the Heritage Impact with Design and Access Statement we prepared in the early 2010s. We modified that document to focus on the proposed extension while retaining the extensive section about the history of service areas. We accompanied the document with sketches and visuals and submitted a pre-planning application. Discussion with a case officer and conservation officer followed, where they endorsed our concept and encouraged us to embolden our all-timber proposal by cloaking it in a more permanent material – leading to the choice of Corten.
How did you make the case in your Heritage Impact Assessment and Design and Access Statement?
As always, we started with tracing the history of the local area. We focused on the period of the particular building to assess its degree of typicality and value to the surrounding locality. This was followed by an analysis of the original design and socio-economic environment of the period, where we evaluated the building’s uniqueness or type by comparing it with the output of the named architect. Finally, we assessed the historic changes and evaluated how they impacted the original design, laying the path for the changes we proposed.
Did this experience give you any other insights which you could share?
We learned to manage clients’ expectations and explain the importance of retaining a degree of flexibility when it comes to the final details of consents and permissions. Before making submissions, we advise clients to inform the neighbourhood of their intentions while we approach local amenity societies, if appropriate.
Perhaps most crucially, we make prompt contact with the case officer and invite an open discussion, wherever possible including the conservation officer.
Sanya Polescuk is founder of Sanya Polescuk Architects
Flint Farm, North Hertfordshire, by WillGamble/Architects.
What is the existing house, and how have you extended it?
Flint Farm is a Grade II-listed farmhouse in North Hertfordshire, with a number of ancillary agricultural barns arranged around a historic farmyard.
The farmhouse itself was in poor condition, with a number of unsympathetic additions that had altered its character and architectural significance. These were removed and replaced with a lightweight contemporary extension that sensitively links the house with one of the neighbouring black barns. Improvements were made to the general arrangement of the historic floor plan while retaining its unique character and the cellular arrangement of the existing spaces.
How did you reach a decision about appropriate size and architectural character?
The brief was to improve the home through an extension that would replace an existing modern conservatory. However, we encouraged the client to extend the property in an alternative location that offered greater opportunities to integrate the neighbouring barn and connect with the surrounding landscape.
The architectural character of the addition is deliberately contemporary in appearance, to create a clear delineation between old and new, while drawing inspiration from its surrounding context so they complement one another. The black metal roof, with its peaks and troughs, references the black timber-clad gables, while its glazed facade creates a lightweight appearance that is subservient to the farmhouse and the wider rural context.
What assessment did you make of the building's special interest, and how did you conserve it?
The building's special interest is defined by its farmyard setting, particularly its close proximity to the nearby barns and ancillary outbuildings. The farmhouse and the outbuildings are characterised by a series of black timber-clad gables.
Internally, its character is defined by a sequence of cellular spaces with an intricate exposed oak frame. Where possible the oak frame was carefully restored, and some sections were replaced using traditional techniques.
During the design process, did you work with a heritage consultant and conservation officer?
The practice undertook a rigorous design process to investigate and test ways of meeting the clients’ brief.
The listed status of the site warranted assistance from an experienced heritage consultant at an early stage to assess the significance of the farmhouse while assisting us in developing the design. This design exercise formed part of a very detailed pre-application submission.
The detailed information and early engagement with the local authority enabled the council’s conservation officer to provide thorough and constructive feedback. A series of site visits and meetings were undertaken with the local authority, the heritage consultant and us, enabling the proposals to be developed collaboratively.
How did you make the case in your Heritage Impact Assessment and Design and Access Statement?
We worked closely with the heritage consultant throughout the early design stages to ensure each design decision was fully justified and the proposal’s impact was carefully and rigorously considered from the outset.
A very detailed Design and Access Statement was then produced, to help demonstrate and justify to the local planning authority (LPA) why certain design decisions were made through diagrams and written commentary. This sat alongside a thorough Heritage Impact Assessment, which carefully assessed the level of harm associated with each design decision.
Have your experiences with listed buildings given you any other insights that you could share?
For listed building applications, we typically engage in a very detailed pre-application process with the LPA to arrive at the best possible outcome for all stakeholders. It requires upfront and detailed design investment from the client in order to get constructive feedback from the LPA to take forward to the planning and listed building application.
Because of the additional time and investment this typically adds to an already lengthy process, a patient client and a proactive design team are often paramount to the project’s successful outcome. However, we feel this process produces the best possible outcome for both the client and building.
Miles Kelsey is an associate at WillGamble/Architects.
Private House, West London, by Donald Insall Associates
What is the existing house, and how have you extended it?
This Grade II-listed house in Chiswick dates back to the 18th century, and was formerly home to the architect Michael Manser. The building evolved to accommodate a basement kitchen and first-floor living rooms, but the ground floor was rarely used. A feasibility study and concept by Alex Lifschutz of Lifschutz Davidson Sandilands identified that the ideal rebalancing to suit the needs of current family life was for a new ground-floor rear extension, with large-format glazed doors enabling the extension to open out to the generous gardens.
How did you reach a decision about appropriate size and architectural character?
An existing light well provided a ‘hard’ constraint on the width, the length and height. We also needed to balance the needs of a modern family space while respecting the historic significance of the existing house. The architectural character of the extension needed to reflect this: a crisp modern addition to suit its time, with a simple palette of materials. The key design move here was the use of oak frames to materially link the extension to the existing timber-framed house.
What assessment did you make of the building's special interest, and how did you mitigate harm?
This building, a mainly Georgian house, survived Chiswick’s transformation from a rural village to an affluent London suburb, with its beautifully crafted principal elevations and interior largely intact. To mitigate harm, we made sure the rear extension was invisible from the riverside setting, and that it touched the existing building as lightly as possible, both physically (by use of a glazed link, which is connected in a reversible way to the main building), but also aesthetically in the transparency of the glazing, so the rear of the house is still visible.
During the design process, did you work with a heritage consultant or other individuals?
We collaborated with our in-house heritage consultancy team to develop the design. Given the sensitivity of the extension on the rear elevation, we engaged with the local authority via a pre-application enquiry before a full application for planning and listed building consent was submitted, to enable us to gain early feedback on the viability of the design. This allowed us to edit the roof form to minimise its impact and improve its visual appearance.
How did you make the case in your Heritage Impact Assessment and Design and Access Statement?
Our heritage consultancy team assessed the significance of the existing house, then set out a commentary on the impact of the proposals, relating back to the relevant national and local planning policy, and feedback gained at pre-application stage. This is important since the 19th- and 20th-century alterations to the house were undertaken with great sensitivity to the special interest of the house, which means the alterations, unusually, enhanced rather than diminished its significance.
Have your experiences with listed buildings given you any other insights that you could share?
Listed buildings need to remain relevant to our times and adapt to mitigate the impact of the climate emergency. The key advice is to understand each building and its significance: there will inevitably be areas that are hugely special and should be cherished, and areas where opportunities for creative design can sit comfortably, and hence can be justified.
Christopher Bell is practice director at Donald Insall Associates