Just a few minutes’ walk away from Old Street, where new glassy towers seem to be emerging from the ground every few months, sits a 1930s former furniture warehouse. Nestled on a mostly residential street in London’s Hoxton district, this brick building is now the home of engineering practice Heyne Tillett Steel (HTS).

 

We’ve long seen architects and engineers take over existing buildings – it’s cheaper and there’s something about the rough, almost unpolished, look that appeals to the creative industries. But 16 Chart Street is neither rough nor unpolished.

When HTS bought the building back in 2019, it had already been converted into an office with many of its key architectural features hidden. It also had planning approval for a stepped-back rooftop extension. HTS brought in Ian Chalk Architects and tasked it with creating a building that would display the engineering firm’s ethos.

‘Our ethos as a practice is to use low embodied carbon materials, and to keep what you can’, says HTS founder Tom Steel. ‘We are less about the operational energy. We wanted a building that would reflect all of that for us.’

The architect tackled this with a series of small interventions: a new entrance, reworking the circulation, and a roof extension. Everything was shot blasted and cleaned up, revealing the texture and detail of the original materials. The brief was to keep as much of the warehouse’s original features as possible.

Viewed from Pitfield Street, the building’s four-storey entrance addition provides a refreshed vista for the street. It has been built over the warehouse’s former yard, creating a covered entrance way, space for a lift core and escape stair, and also houses meeting rooms on the upper floors. The building’s old staircore and entrance at the opposite end of the building were deemed too awkward to keep, and have been replaced with toilets on each floor. This new addition marries with the old warehouse through the openings originally used to load furniture, materials and equipment into the large warehouse floors. These have been left open – some with their original large timber doors – connecting the open-plan office spaces to the meeting rooms.

A new perforated steel stair cuts through all the floor plates. Positioned at the far end of the office, it gives a focus to the open-plan space. Its perforations were carefully modelled to allow light to penetrate the building. The steel is utilitarian yet crafted. The view upwards from the bottom of this stair also reveals the different layers of the warehouse, from the polished concrete floor of the basement to the beam-and-block floors above, and finally to the CLT of the roof level.

At the upper level the original structure’s cast-iron columns and beams have been replaced with steel in order to hold up the new CLT roof extension. Instead of adding the setback roof extension already approved by planning, the architect added an extra floor in line with the building frontage. ‘The setback would have been subservient,’ comments practice founder Ian Chalk. Instead, the brick parapet was built up by about a metre to support the CLT roof extension.

Here, hardwood glulam beams span across the 10m width of the top floor, leaving a huge free space that seems particularly generous. It’s a space where staff eat lunch together, where events will happen when things begin to open up a little more, and where the office can meet as a whole in one space. The new sawtooth roof acts as one of the building’s identifying features, combining new with old and responding to the surrounding roofscape.

 

Windows can be divisive on retrofit projects; there is always a trade-off between energy efficiency and the look of the building. Here, the original windows had already been replaced with uPVC sometime in the 90s. These were now replaced by Crittall windows, which were a lot more in keeping with the original 1930s windows. Where new windows have been inserted, the architect used the existing opening widths, and just lowered the sills. This is evident in a large new opening in the entrance way. As you walk up the steps to the office, you get a glimpse through this large pane of glass to the bicycles hung up and the desks beyond. It is also all naturally ventilated. This feels right for an office in the age of Covid. Windows can simply be opened to let in fresh air without the need for complicated ventilation systems. The south façade is also relatively solid in order to prevent any overheating. The windows that are here, however, are causing issues with reflections on the meeting room’s screens and it’s likely that shutters, blinds or curtains will be added in order to control this.

With a new office and the pandemic has also come a new way of working. In September everyone came back to the office. It was the first chance to test out the building in full occupation. With its staff partly working from home and a new clear-desk policy in place, HTS has just 120 desks for 130 staff. There are drawers at the end of each office floor, and each staff member must clear their desk into their drawer at the end of the day. It’s reminiscent of classroom drawers from my primary school days, but for the staff I talked to, it seems to work. There has also been some adaptation in response to the challenges of Covid. In the basement space, booths have been added for Zoom calls and online meetings. These are always in use and there are now plans to add more.

This careful retrofit strips the building back to its core materials. For HTS it acts as a platform to display its work – both for clients but also for young engineers learning the ropes. Here, they can see retrofit in action. ‘We wanted a showcase of structural elements,’ says Steel. ‘It was very important to us to show how we have reused the original building. We are able to show clients what we do through visiting our office.’

But there is also a level of future-proofing in the design. The building could be broken up and tenanted in the future if the firm’s situation changes. And they are monitoring the building’s energy use in order to understand how much heating and cooling they are using. There is clearly the idea that this is a continuous learning process and that there will be adaptations and tweaks to the building as they learn to occupy it.

This is an analogue approach to retrofit; it wasn’t about big invasive choices or grand gestures. There are no flashy materials or complicated building management systems. But it is also a building which couldn’t have been achieved without an enlightened and informed client. Unusually for a building of this size, it was realised through a traditional contract. And for those that reminisce fondly about traditional over design and build, the quality of finish and detailing of this retrofitted warehouse will add fuel to that argument.

Client’s view

This is our fifth home in our 15-year history, during which time we’ve grown from three to nearly 140 people. This was our opportunity to design the building we really wanted to spend time in and take everything we’d learned from our earlier offices.

We were gifted with a robust, detached building with a lot of character, natural light and ventilation on all elevations.

We wanted it to be a place that made the passage through the working day effortless. Jump out of bed, roll or run to the office and then have every facility here for you, arranged in a seamless logical flow: bikes, showers, exercise, food, space, light, connectivity and collaboration; spaces and surfaces that we have spent the best part of three years honing. This is a place that stands for what we believe in: low-carbon design, reusing structure and a culture of collaboration and learning.

Just as a piece of second-hand furniture has a character and charm that is rarely found in the newly built, I think the same applies to our building. The constraints and discipline of working within an existing frame have created the warmth we already feel for it.

The fact the pandemic happened during construction allowed us to amend the environment to suit the emerging way of working, with plenty of spaces to make noise yet not disturb people in what is an open-plan office.

What Covid took from our way of working, we have grabbed back better than ever with our new home providing community, culture and inspiration.

We wanted to make the office somewhere where people would choose to be. Mark Tillett, founding director, Heyne Tillett Steel and part owner, CSI Investments

 

Architect’s view

The project started with the key decision to reuse and extend the existing building; demolishing only what was necessary and enjoying its unique character. Cross laminated timber (CLT) was used for the majority of new structure, with concrete used to form the basement for practical reasons.

The building employs a practical and ‘analogue’ approach to sustainability, quietly outperforming RIBA and LETI targets through minimising technological obsolescence and accepting the imperfect nature of raw materials as a finish. Repairs to fabric were restricted to structural repairs and new finishes used sparingly; minimising material waste and maximising future reuse and recyclability.

Natural ventilation is used throughout the building, with heat recovery systems used in the lower ground floors. At third floor, the CLT studio space forms a highly efficient warm lid to the building, with the north-lit space maximising daylight while minimising solar gain.

The decision not to insulate the existing masonry walls, in addition to wanting to avoid a loss of character, was seen as a balance between the embodied carbon of all the associated components and the potential carbon saved through heat retention over the operational life of the components.

The outcome of this is a rich patchwork of exposed masonry scars interspersed with moments of contemporary detail which enjoy the idiosyncrasies of construction. The existing red brick, terracotta pots and concrete beams form an uncomplicated and earthy palette combined with a raw steel stair, galvanised steel windows, exposed floor tiles and timber. The didactic material expression extends into the detail with welds, connections and services all carefully co-ordinated, allowing them to be enjoyed. Giles Wheeldon, associate, Ian Chalk Architects

Engineer’s view

Often our first move on a refurbishment project is to carry out extensive investigations into the building’s structure and foundations using archive drawings, maps and surveys. At Chart Street, our understanding of the history and character of the existing building allowed us to explore its full potential for reuse, ultimately reducing demolition works and embodied energy costs.

Once investigations were complete, we worked with Ian Chalk Architects on the complex redesign of the existing building. A key structural intervention involved remodelling the top floor to create a large open space for meetings and events. Constructed from CLT and glulam, the new floor level has been designed as a flexible, column-free space with four rows of 10m-long north-light windows to maximise daylight. A vertical lightweight CLT extension has been designed to the side of the building to complement the new sawtooth roof structure, alongside a new stair and lift core and a new covered entrance to the building.

When designing with timber, additional consideration is needed for the exposed connections and articulation of the joints to ensure consistency across a surface, ie alignment of CLT panels along an exposed soffit to ensure a considered design aesthetic. Another key interface is between the CLT floor panels and perimeter steel angle supports, where construction tolerances that could create an unappealing step between elements were overcome by the introduction of a 10mm recessed shadow gap, creating a clear separation of elements. To support the north lights and CLT panels, hardwood BauBuche LVL beams span the width of the top-floor space, providing the natural strength to carry the sawtooth roof form as well as delivering beautiful, clean finishes. Andrew Howe, associate director Heyne Tillett Steel

Working detail

Devised as a contemporary version of the found building, the new addition was designed to be an expression of its construction and sequencing. Left uncovered, the new materials mimic the rawness of the materials below. Concrete beams, terracotta pots and exposed brickwork sit alongside lacquered and galvanised steel, timber and resin.

The worm’s-eye detail shows the relationship between the new north-lit studio and the existing floors below.

A new steel structure at second floor provides support for exposed CLT slabs to span between and form the soffit finish; the combination chosen to mediate the transition between the old and the new. Above, bolted connections fix glulam columns to the new beams through precise pre-cut sections in the slab panels. Notched triangular-shaped panels span between the glulam columns, allowing each 10m-long BauBuche beam to sit in place before being locked in by the next panel. Finally, single roof slabs span between triangular panels, supported in the north-light by steel T-sections.

At upper levels, cabling is concealed behind a small upstand along the length of the beams with light fittings integrated into the structural steel Ts allowing a continuity of lighting direction between day and night.

Deep rebates express the joints between panels and a continuous datum, informed by the efficiency of manufacturing, runs around the space forming the heads of windows and openings.

The residential grade CLT is softened with a light whitewash, combining with the finer grain of the BauBuche beams and delicate detailing of the raw steel stair in an attempt to emulate the quality of a bespoke piece of joinery. Giles Wheeldon, associate, Ian Chalk Architects

Project data

Start on site August 2019 Completion July 2021 Gross internal floor area 1,460m2 Construction cost Confidential Architect Ian Chalk Architects Client CSI Investment/Heyne Tillett Steel Structural engineer Heyne Tillett Steel M&E consultant Peter Deer Associates Quantity surveyor Exigere Project manager Stature London Principal designer MLM Approved building inspector MLM Main contractor Conamar Building Services Sustainability consultant KLH Sustainability Planning consultant Iceni Projects Workplace consultant Spacelab Interior designer Spacelab Fire engineer MLM Rights of light GVA Party wall surveyor Brook Vincent CAD software used MicroStation, AECOsim Building Designer

Performance data

Percentage of floor area with daylight factor >2% 84% Percentage of floor area with daylight factor >5% 5% On-site energy generation 0% Heating and hot water load 6,187 kWh/m2/yr Total energy load 11,695 kWh/m2/yr Carbon emissions (all) 34,583 kgCO2/m2 Annual mains water consumption 0.05 m3/occupant Airtightness at 50Pa 8.26 m3/h.m2 Overall area-weighted U-value 0.96 W/m2k Embodied carbon Total 360 kgCO2e/m2 (equivalent to 614 tonnes CO2e) Operational carbon (over 60 years using FES steady progression 2019 scenario) 728 tonnes CO2e Whole life carbon As per carbon emissions (all) Predicted design life in years 60 years Annual C02 emissions Varies from year to year as grid is decarbonised