Challoner House is a renovation project, converting and extending a locally listed building into high-quality office space for an owner-occupier using natural and recycled materials.Originally comprising two townhouses and an iron printing press, the building is situated in the heart of the Clerkenwell Green Conservation area, renowned for its architectural and historic interest. HUT was briefed to sympathetically transform, extend and thermally upgrade the existing building while preserving the buildings character and history.The project commenced by undertaking studies to understand alterations to the building from the original plan form through to its current layout. The original plan form is mapped out in brick tiles within the new offices. Brick arches and historical patina are left exposed and expressed within the renovation works. A section of flooring at ground floor has been removed to create a spacious town hall, which connects to the basement. Door and window openings from the original townhouse remain suspended along the elevation at high level to preserve the buildings heritage and contribute to its story.Alicia Sardi, senior architect, HUT ArchitectureAdvertisementProject dataStart on site September 2022Completion Gross internal floor area 2Net internal floor area 1,228m2Construction cost UndisclosedArchitect HUT ArchitectureClient AkeliusStructural engineer Morph StructuresCivil engineer Morph StructuresM&E consultant Harley HaddowQS Johnson AssociatesProject manager Johnson AssociatesPrincipal designer PeligroApproved building inspector QuadrantAcoustic consultant Clement AcousticsBREEAM consultant Build EnergyFire engineer Semper FireMain contractor Copper ProjectsCAD software used RevitAnnual CO2 emissions (predicted) 43.45 kgCO2/m2/yrArchitects choicesLime hemp plaster was used to thermally upgrade and line the existing solid brick external walls. Selected for its breathability, thermal properties and flexibility, it was ideally suited to the undulating original brick walls in order to improve airtightness. Made from a renewable plant fibre, lime hemp plaster locks in CO2 and has a lower embodied carbon than traditional plasters.Selected for its fire-resisting properties and recycled content, Fermacell has been used where possible in place of traditional gypsum board. We actively seek to specify materials that consider their environmental cost and Fermacell has a carbon-negative production process, with 100 per cent recycling from cradle to gate.Alicia Sardi, senior architect, HUT ArchitectureSpecificationThe building was originally constructed from brick and timber. Steel structural members were added at a later date when the individual buildings were amalgamated.AdvertisementThe envelope had little to no insulation and required significant thermal upgrades while preserving the historical fabric. Breathable materials sheeps wool, wood fibre board, hemp and lime render were selected due to their low embodied carbon, thermal benefits and breathability. Internal walls were constructed using Fermacell, selected for its recycled content, moisture control and fire-resisting properties. The existing steel structure has been reused and repurposed to avoid unnecessary waste. The new butterfly roof is constructed using a glulam ring primary structure, with the vaulted, sculptural form being expressed; creating a strong visual connection to the form and heritage of the original building.Internally, original door and window openings have been left in situ. While they no longer serve a functional purpose, they visually describe the buildings previous uses and layout. The external faades have been upgraded and now feature double-glazed, timber-framed sash windows in keeping with the heritage while vastly improving the buildings thermal performance.Alicia Sardi, senior architect, HUT ArchitectureSelected productsLime hemp plaster wall liningT-MawrLime hemp plaster wall liningExternal wallslime.org.ukTimber sash windows with slim glazingOriginal SashThroughoutoriginalsash.co.ukAluminium rooflightsGlazing VisionPitched and flat Roof Lightsglazingvision.co.ukSliding doorsAssa AbbloySlim-Eco 2 Sliding DoorMain entranceassaabloy.com