Kelly Harrison, Director at Whitby Wood, explains how using timber to add storeys to a building could add transformative amounts of space to major cities like London.
Optoppen is a Dutch word which translates as “topping up”. It is also the name of an online resource that will help anyone interested in airspace development – adding storeys to a building’s roof.
Using timber to extend upwards makes sense: it is a much lighter construction material than most. This means it can potentially add storeys without the need for strengthening the building’s foundations. The Optoppen website hosts an interactive tool to help clients identify the opportunity of their site for additional floors. This tool allows asset owners and urban planners to quickly assess the potential a building has for a rooftop extension.
It provides a high-level structural assessment and reports the possible amount of floor space that could be created – together with the amount of carbon sequestered in the mass timber, as well as the structural embodied carbon of the construction.
It also provides useful examples of existing Optoppen extensions – it now has a fascinating library of timber-based rooftop extension projects from a healthy mix of countries. You can also read about the various policies in Europe that are relevant to this kind of project.
Exciting opportunities for the world’s rooftops
One intriguing part of the Optoppen site is its interactive ‘City Opportunities’ page. There you can read about the new units that timber rooftop extensions could create in the cities of London, Rotterdam and Barcelona. The existing buildings in those cities were assessed in terms of numbers, heights and typologies; numbers were crunched; and some promising conclusions were reached about Optoppen’s potential.
In London, Optoppen could deliver 515,000 new flats. That amounts to 45,752,900m2 of internal area: an increase of some 7.7%. The timber used would sequester 4,330,400 tonnes of CO2e – , that’s the equivalent of over 5.5 million Sitka Spruce trees. Similar studies found that up to 125,000 flats could be added to Rotterdam’s rooftops; and over 195,000 to Barcelona’s.
Optoppen’s potential for unlocking London
London has a hugely varied range of building typologies, from low-rise terraces to Edwardian mansion blocks, to more modern office buildings and high-rise blocks of flats. All of these typologies have potential for adding storeys, with different boroughs presenting different scales of opportunities.
The study breaks that 515,000 total down by borough. You can select a particular London borough to see at a glance what Optoppen gains could be made there. Southwark could gain 24,800 new flats (2,010,200m2), while Westminster could gain 48,300 (4,059,200m2).
The assessment of all this for London was based upon available information on parameters for urban density, for each borough, to assess opportunities for adding flats to those over 3 floors.
“We took our data from public information published by the Greater London Authority (GLA) on buildings in each London borough,” explains Kelly Harrison.
“Using information on building heights, we could determine the number of storeys. We then used available statistical data on each borough’s building use and typologies.”
“A borough might be comprised of 20% residential and 80% commercial or industrial, for example. For each borough, we took the average building height and number of storeys for each use type, to estimate how much timber you could add on top using the Optoppen tool.”
The information is therefore based on a few assumptions, but with some conservatism, the overall picture that it gives remains a reliable estimation.. “For Rotterdam, we could calculate that Optoppen could provide a 16% increase in units,” Kelly points out. “But for London, we had to be very conservative in our assumptions, as the data wasn’t as good , and the size of the city much greater.”
The conclusions about London’s Optoppen potential were based on buildings of three storeys or more, and less than 20. That is because buildings lower than that would likely require foundation strengthening, and those over could give large additional area results, which could be unreliable and skew results.. “That limitation assumes we don’t touch any foundations,” Kelly explains. “You could add many more if you were strengthening or adding new foundations, but this adds embodied carbon and is almost impossible to quantify, so is a limitation of the study.”
Proving that it can be done
While the Optoppen website does talk about these numbers in terms of “flats”, the point is more about the sheer amount of new useable space that Optoppen could provide. “Really, it is about adding a percentage of floor area, but talking purely in terms of square metres is not necessarily meaningful to everyone,” Kelly says. “So we’ve tried to contextualise it for people by suggesting how many twobedroom flats that space could be.”
The particular use, and the viability, of an Optoppen project will vary from area to area. “Viability is of course a crucial part of any development,” Kelly states. “The whole point of the Optoppen website is to demonstrate that it can be done: to show what it could mean for a building asset in terms of unlocking all that space. The tool helps early decisions making, to give a clear brief to the design team, who need to all be on the same page to make this work, as found in our stakeholder roundtable discussions. The case studies give precedent, and the deep dive research studies in the Netherlands show how you can overcome the detail. We have recently also included a Swedish Design Guide, also translated to English to give further detail.”
Kelly suggests that councils in particular are increasingly interested in this work. They have ambitious new housing targets, and many of them are already stretched in terms of need. The organisation Trust for London has highlighted that every London borough (bar Hounslow) has a higher proportion of households in temporary accommodation (TA) than the England average.
In nine London boroughs, the proportion of residents in TA is five times the rest of England. For a council, adding storeys to a four-storey council block would be much more cost-effective than the enormous monthly costs of housing those without homes in TA or hotels.
Kelly also points out that in areas such as Central London, the combination of conservation areas and an increasing presumption in favour of retain and retrofit, is making it hard for a developer to demolish a building. For private developers looking at commercial development, building upwards will inevitably step into the breach, especially in areas where the values are highest.
To learn more, visit the website www.optoppen.org
