Hawley Mews

Hawley Mews is a development of three terraced houses on a very confined site in Camden, London. It demonstrates how the objectives of the Mayor’s Draft London Plan 2017 can be met with care and ingenuity. The project won the Award for Excellence in Architectural Technology accolade at the CIAT AT 2019 Awards.

[edit] Introduction

Formally a car park, the site was surplus to Camden Council’s requirements and was acquired at public auction in 2013. The total site area measuring 280m2 was heavily overlooked by neighbours, a mere 7m from both boundaries which presented a difficult but exciting design challenge. Planning Permission was obtained by Goldcrest Architects in 2014.

[edit] Technical challenges

healycornelius design consultancy was engaged to develop the technical design from receipt of planning permission through to completion and handover.

The first technical challenge, rather unusually, was to build a full-sized model of a single bay of the property (erected and removed in a single day) to demonstrate the actual massing relationship of the proposal to the adjacent buildings. The full-sized model demonstrated how the roof form is modelled and shaped to slope away from boundaries, creating a respectful sculptural massing and volume to the adjacent properties.

The delivery of a high-performance development defined the goal, meeting with the local planning authority’s requirement for the Code for Sustainable Homes – Level 4, also achieving a minimum EPC Rating B through the considered specification of materials.

Each house is arranged around a private south-facing courtyard, extending the open-plan ground floor living space. Large glazed doors open onto the courtyard garden, increasing the sense of light and space with windows in all the rooms. A large opening skylight above the stair allows daylight into the depth of the plan and natural stack ventilation in summer. The staircase becomes a gallery space, library and home office. Each house is carefully detailed and designed to maximise light, space and storage.

The houses consist of two bedrooms on the first-floor level, one with an en-suite. All comply with Lifetime Homes Standards where an accessible WC is located on the ground floor, designed to convert to a level-access shower room. Accessibility is maintained through the allowance for the retrofit of a platform lift to the entrance from the front path, as well as a through-floor lift within the house.

[edit] Resolving site constraints

To reduce the scale and impact of the development, the ground floor was set at 500mm below ground level. This presented a challenge in terms of the depth of excavations and a redundant drainage culvert running through the centre of the site. A cantilevered foundation design was adopted which sat on two long deep-trench foundations positioned on each side of the culvert. The retaining walls were constructed using concrete-filled cavity blocks with a type C waterproof tanking system installed to prevent moisture ingress.

A number of modern methods of construction for the external walls were evaluated to address the requirements for a high-performance fabric, maximising floor area on the confined site and to meet the construction challenges of the restricted site area. The client preferred to adopt a traditional masonry method of construction. Accordingly, a 200mm autoclaved aerated concrete (AAC) block wall was selected using a thin-joint technique with 2-3mm special adhesive joints. External wall insulation (EWI) and a polymeric render finish delivered a U-value exceeding the minimum requirement.

[edit] Wall construction

This method of single-skin masonry assisted in reducing the overall programme compared to traditional construction methods. When the project reached construction, a key raw material of the AAC blocks, pulverised fuel ash, was in limited supply. To address this challenge, we sourced an alternative aircrete block which required a conventional mortar joint. Following analysis, the specification adjustment did not affect the U-value, however it had a slight impact on programme as traditional mortar joints were constructed in limited lifts.

The 200mm AAC blocks – which were selected for the external fabric – contributed to the sustainability rating. The thermal mass of the blockwork retains and emits heat into the internal spaces, the application of EWI with polymeric render delivered a performance above regulation requirements.

[edit] Green credentials

Each house has a bio-diverse, living, green roof to provide diverse plant communities, increase energy efficiency, minimise rainwater runoff volumes and enhance the appearance for those overlooking.

Rainwater is harvested from the zinc roofs to a water storage tank in the courtyard to water plants etc. Fire protection measures are implemented using a mist suppression system which statistically delivers a greater likelihood of survival in the event of fire. Mist suppression was a requirement to meet Part B of the Building Regulations.

The primary space heat demand is met by an energy-efficient air-to-water heat pump serving underfloor heating throughout both floors. A mechanical ventilation with heat recovery system is installed in each house resulting in controlled ventilation and negation of the requirement for trickle vents. Focused attention to detail ensured that the design of all junctions was carefully addressed, resulting in an air permeability of 4m3/h/m2 @ 50Pa.

The bespoke folded metal screen, incorporating the pedestrian gated entrance, defines the threshold between the public domain and the private realm. It is designed to be tactile, feel safe, secure and substantial as you move through it. Inspiration for the screen design was drawn from the historical use of Hawthorn trees to enclose land and the folds in the metal, which echo the texture of shipping containers, refer to Camden history as a transport hub in the era of the canal network.

The resulting abstract perforated pattern which echoes the Hawthorn reference is fully revealed at dusk when the backlit screen illuminates Hawley Mews and positively connects with the local street art. Careful coordination to balance the challenges of the fabricator, main contractor and the artistic guidance of the client’s team was required to achieve the result.

[edit] Judges’ comments

Hawley Mews is a neat and clever site design and layout. The scheme demonstrates the true flexibility of architectural technology, with its innovative technical solution — a high quality solution for a constrained site, maximising natural light. The evidence of compliance with planning requirements was managed in an innovative and transparent way and with a sensible approach. The fire-engineered mist suppression solutions, to ensure safe results, overcome seemingly impossible site constraints.

[edit] About this article

This article was written by healycornelius design consultancy. It was previously published in the Chartered Institute of Architectural Technologists’ (CIAT) AT Journal, Issue 131, Autumn 2019 and can be accessed HERE.

Other articles by CIAT on Designing Buildings Wiki can be accessed HERE.

[edit] Related articles on Designing Buildings Wiki

• Approved Document L.
• Biophilia.
• Building regulations.
• Building Research Establishment.
• Cool roofs.
• Designing green and blue roofs.
• Energy performance certificate.
• Flat roof.
• Garden.
• Green walls.
• Insulation.
• Kappa value.
• Living Roofs and Walls, from policy to practice.
• Mews.
• Rain garden.
• Simplified Building Energy Model.
• Standard Assessment Procedure.
• Sustainable urban drainage systems.
• Thermal bridge.
• Thermal mass.
• Types of roof.
• U value.

--CIAT