Led by engineering consultancy Whitby Wood, a group of organisations -- New Urban Networks, Holland Houtland, Creative City Solutions, Institute for Advanced Architecture of Catalonia (IAAC), Mule Studio, and Rising Tide – has been funded by European bio-based network and fund, Built by Nature, to develop important resources and tools for Optoppen to grow.

Central to the new platform is an interactive and dynamic tool that allows building owners and city planners to understand quickly and easily the vertical extension potential of their buildings, providing a high-level structural assessment and reporting the possible amount of floor space area and carbon emissions stored in the mass timber (as well as the embodied carbon of the Optoppen works themselves).

This website will also include a global database of Optoppen case studies; an analysis of European countries’ policies; and news about launch events and workshops in the UK, the Netherlands and Spain.

Optoppen projects will focus on common building types in European cities, with a focus on the UK, the Netherlands and Spain, with estimations available at both building and city levels.

"Optoppen," a Dutch term translating as "topping up," refers to a construction method aimed at maximising the usable space in our cities through the addition of lightweight, low-carbon roof extensions. Many existing buildings have excess structural strength and can happily accommodate additional storeys. Optoppen not only helps address the shortage of housing in a climate-sensitive way, but also offers solutions to financing renovations and energy efficiency improvements: adding new living or working spaces on top of existing structures creates increased property value while avoiding the carbon cost of demolishing and building new.

Optoppen is a flexible solution, and can be applied in several ways – for example, as:

- ‘Reinforce-Optoppen’: This method involves adding a robust table-like framework that supports the new storeys. It can be used to add a significant amount of new floorspace, sometimes reshaping the building's entire silhouette.

- ‘Transformation-Optoppen’: This involves a comprehensive overhaul of the building, sometimes transforming its purpose, and massing, while adding new space. It is ideal for extensive renovations that aim to repurpose or modernize older structures, with the added space paying towards the refurbishment.

- ‘Occupied-Optoppen’: Focused on minimising disruption, this technique uses modular construction to add floors quickly and quietly, allowing residents to remain in the building during renovations. This method is particularly beneficial for occupied residential buildings needing updates.

Overall, "optoppen" provides a strategic response to urban housing challenges, promoting denser, more efficient use of space while advancing sustainability and energy conservation in urban environments.

From 2020 to 2060, the world is expected to add 241 billion m2 of new floor area to the global building stock — equal to adding a city the size of Paris to the world every week for 40 years.* As our cities grow, we need to find low-carbon ways to provide new housing and workspace. One way is by adopting ‘sufficiency’ thinking – making the best use of space.

- Optoppen gently densifies existing neighbourhoods, avoiding carbon-intensive and disruptive demolition and rebuild

- Using timber for optoppen (instead of carbon-intensive materials) reduces the ‘upfront’ (embodied) carbon cost of the development, and as a light material, it makes the most of the existing building’s structural capacity

- Optoppen can be a way of funding improvements and upgrades to the building, improving existing residents’ quality of life

- Timber stores the carbon it has sequestered during its growth. Using timber as a building material means these harmful emissions are locked away for decades.

Building owners, asset managers and real estate investors who want to increase the value of their portfolio but want to avoid environmentally-damaging construction methods and materials.

Town and city planners who need to increase housing and office space for their changing populations, yet also have carbon reduction targets to meet.

Residents whose building needs renovation, or who want energy efficiency upgrades – but want to remain in their building.

Developers with commitments to regenerating urban locations in a climate-neutral way.

Architects and engineers who want to bring better solutions to their clients.

Academics and students who are analysing how we can meet both urban growth and emissions targets, and new ways of planning our cities.

Vertical extensions are an established way to expand a building – but building up in timber is relatively new. As climate concerns become a driving force in the property world, Optoppen is now starting to take off around the world, with the Netherlands, the UK and Spain leading the way.

The UK is seeing owners of commercial buildings add new storeys to increase lettable floorspace - such as at Gramophone Works and at Technique.

According to Skyroom research on vertical extension, joint research by engineering firm WSP and UCL suggest that there is capacity to build 630,000 new homes on top of London’s municipal buildings. Knight Frank suggests there is scope to build 41,000 rooftop homes in central London alone; and the architecture firm HTA Design has identified capacity for precisely 179,126 new homes.*

In the Netherlands, the Ministry of Internal Affairs’ research on Optoppen estimated that around 100,000 additional homes could be added by 2030: “Two-thirds of the potential can be realised by topping up multifamily housing built in 1965 or later, because these houses are structurally suitable for this and because in many cases they are owned by housing corporations.” 

In Spain, research from The Open Construction and Building Technology journal focused on the Eixample area of Barcelona shows that approximately 2,500 in Eixample are capable of accommodating vertical extensions. These buildings provide approximately 800,000m2 of buildable space. At average dimensions for the city centre, this represents approximately 90,000 to 130,000 new homes.

Structural: An initial appraisal can provide a basic understanding of structural constraints using conservative assumptions

A more detailed assessment will require additional information of the existing building such as: archive records, foundation/superstructure investigations,  material testing etc. to allow the extent of any structural intervention to the existing building to be confirmed and refined 

Building policy: including permitted height, safety regulation, permits/consents needed, design and aesthetic

Architectural requirements: Unique requirements of a building in relation to fire protection, access requirements etc.

Optoppen policy is almost always set by city administrations.

Our project is creating a knowledge hub of cities’ Optoppen-related policies. If you would like to share yours with us, please get in touch.

For an Optoppen project, here are the main financial and regulatory areas where attention is required:

Structural Analysis: Ensure the existing building can support the additional load. This requires specialist engineering assessments. If the existing structure cannot support the new structure, renovation of the existing foundation could be necessary, which can be complex and costly. 

Permits / Planning consents and Building Regulations: Normally an Optoppen project of just one or two storeys can be included in light-touch renovation permits, depending on the region.  Attention needs to be paid to local zoning laws and building codes that impact exactly what can be constructed.

In addition, pay attention to adherence with all relevant building regulations, which may have changed since the original structure was built.

Design: The design must align aesthetically with the existing structure while meeting modern standards, and the limitations of the material and MMC processes, potentially requiring more intricate architectural planning.

Construction logistics: Consider at an early stage the challenges of transporting and assembling materials in potentially constrained urban spaces without disrupting the current building’s use – especially if residents remain in situ. 

Ground Rent (Nieuwe Erfpachtuitgifte): If the property is on leased land, an Optoppen project might require renegotiation of the lease or issuance of a new lease.

The Optoppen tool allows building owners and city planners to understand quickly and easily the vertical extension potential of their buildings, providing a high-level structural assessment and reporting the possible amount of new floor area and carbon stored in the mass timber.

Launching in autumn 2024, it will be a free to use web-based tool.

It has been built by Mule Studio with input from project partners.

Autumn 2024