Timber on top – could Optoppen start a rooftop revolution?

CGI image of rooftop extension of Arding & Hobbs building, showing interior of upper floors

If we want to build more sustainably, we should look up. That’s what the Optoppen project is suggesting, showing us the potential of timber to provide low-carbon rooftop extensions.

It is often said that the most sustainable building is the one that already exists. If we want to give our cities and towns more useable space, we need to make the most of the buildings we have – adding to them, instead of always building from scratch.

Example of the interactive design tool that Optoppen is developing

Optoppen is developing an interactive tool to help designers and clients realise the rooftop potential of a building.

Extending upwards is an excellent way to do this – delivering new floorspace on the top of an existing building. Optoppen (meaning ‘topping up’) is a new, ongoing project that shows how to increase the useable space in a building through lightweight timber roof extensions.

A tool to help transform our rooftops


The CLT-based extension at Lower James Street, in London’s Soho, was designed by Henley Hale Brown architects and engineers Heyne Tillett Steel.

It’s an international venture – engineers Whitby Wood have joined forces with Mule Studio and Rising Tide in the UK, New Urban Networks, Houtland Holland and Creative City Solutions in the Netherlands, and the Institute for Advanced Architecture of Catalonia (IAAC) in Spain. Funded by Built by Nature, they’re developing an online resource platform and interactive tool to help designers and their clients identify additional areas for building ‘up top’.

This tool will allow asset owners and urban planners to quickly understand the potential a building has for a rooftop extension. It will provide a high-level structural assessment and report the possible amount of floor space that could be created – together with the amount of carbon sequestered in the mass timber, and assess the economics.

It will also show useful examples of other Optoppen extensions, while also explaining the various policies in Europe that are relevant. Common building types will be covered, with a focus on the UK, the Netherlands and Spain.

“Many asset owners, developers, investors and city governments are simply not aware of the potential of these ultra-low-carbon rooftop extensions, and this project hopes to bolster understanding of this exciting opportunity”
Paul King, CEO, Built by Nature

There’ll be an opportunity to learn more about it all at the UK Timber Design Conference on Wednesday 26 June. The conference’s ‘Timber On Top’ session will feature Kelly Harrison (Director, Whitby Wood) in discussion with architect Nahdya Thebalt (Associate, Stiff + Trevillion) and Joe Giddings (UK Networks Lead, Built By Nature).

A CGI of the interior, with CLT clearly visible at ceiling level.

A CGI of the interior of the mass timber roof extension at the Arding & Hobbs building, a former department store.

They’ll be sharing insights from the many innovative ‘timber on top’ topics they’ve worked on. Tickets are now on sale.

With Europe’s cities facing multiple housing crises, Optoppen is one solution that avoids demolishing a structure: enlarging floorspace with a low-carbon material that itself stores carbon.

Send us your examples

Timber Development UK is helping the Optoppen project to source case of studies of vertical extensions in timber. Please contact us at info@timberdevelopment.uk if you have a project that should be featured.