24 November 2021

Expanding into the airspace: why engineered timber is a good fit

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The new Permitted Development Right (PDR), which came into effect on 1 August 2020, enables property owners and freeholders to extend upward into the airspace by two storeys without the need for planning permission. The value of extending into the airspace is increasingly being recognised by developers, and the sale, lease or transfer of ‘airspace rights’ is becoming big business.


This is an important trend, particularly for major cities where available land is limited and values tend to be high. The 2017 Greater London Authority’s London Strategic Housing Market Assessment identified a need for 66,000 new homes a year in London, but the reality is that this need is not being met. Airspace developments can help to meet the shortfall in new housing, also addressing issues of high-density living and the efficient reuse and adaption of existing structures.


Benefits and challenges

Expanding into the airspace is a good way for building owners to maximise the return on their investment. However, there will be some issues for project teams to be aware of that are specific to airspace developments, including the questions of weight and how to interface with the existing structure.


As a mass timber and hybrid construction specialist we have recently delivered several cross-laminated timber (CLT) airspace schemes and there are generally logistical constraints around site access, material deliveries to site and working on the rooftop of a potentially occupied residential building. This is where CLT comes into its own; CLT is a lightweight material, highly accurate and efficient due to prefabrication, and its use ensures speed and ease of construction with zero waste. Operating on just-in-time principles, the number of deliveries with CLT is minimised, as is disturbance to neighbours and the pre-cut panels can be lifted on by crane, which is a major benefit due to the constrained nature of many rooftop sites.


Commercial extensions

Commercial buildings provide an ideal environment for the addition of two or more storeys. In 2016 Lancelot Homes used CLT to create eight duplex apartments on the roof of a 1970s, three-storey brick built, mixed-use commercial and residential building in Kingston-upon-Thames. Originally purchased by King Street Group in the early days of the permitted development rules, the scheme was designed by Stephen Davy Peter Smith Architects to Level 4 of the Code for Sustainable Homes (now defunct).


‘From the outset the design intent was to develop generously sized residential units that would be compliant with the London Plan should the client wish to sell it on, for example to a housing association,’ says Architect, Stephen Davy. The original owners in fact sold the site to Lancelot Homes and the rooftop apartments were developed for private sale while maintaining the London Plan space standard. ‘Lancelot homes were keen to use CLT for the scheme and we were familiar with CLT and keen to use it, too. The building structure and layout could accommodate CLT and it helped resolved complex roof geometries at the rear of the scheme where a small neighbouring courtyard raised some right of light and sunlight issues. The fact that the panels are formed in highly efficient factory conditions and the elements cut to fit allowed us to get an envelope up quickly, including the complicated roof pitches which were then overclad. This was far easier than installing a steel frame then having to adjust and work the geometries and fill the gaps between.’


‘The great thing about this building was that, although only three storeys in height, it was a concrete frame, a pretty solid structure for that scale of building, which meant adding the two storeys on top wasn’t an issue. A commercial building is generally designed to take heavier loads and, by using a lightweight structural material such as CLT, the potential is there to add storeys. However, some more modern commercial buildings may have a substantial frame, but the nature of the build could make it trickier; for example, with a steel frame office building you would have to consider more carefully the detailing and isolation for sound transfer between units.’


Residential extensions

The private residential sector is also seeing an increase in the number of airspace schemes, both single and multiple units. ‘There’s a huge opportunity for airspace developments in the private residential sector, as long as they’re done well,’ says Mark Stevens of HASA Architects. ‘The development needs to be responsive to the typology and context. I think we could develop airspace typologies, for example, with modularised systems or a choice of typologies that can work on Edwardian or Victorian terraces. This could certainly address the shortfall in housing, as well as giving people the option to grow instead of having to move.’


HASA has recently completed a 25m2 rooftop extension for a private client in south London. The home is an end-of-terrace house, part of an original boarding school that was converted in 2000 into six semi-detached houses. ‘The client wanted to achieve an en-suite master bedroom with roof terrace and to get the maximum amount of light possible to the property. They had a relatively limited budget and wanted something that would be quick to erect, so we took them through the path of looking at a CLT prefabricated structure that could sit on the existing building. We used CLT in its raw state with a light white wash.’


‘One thing we learned was that if we were to do a similar project we would partner up earlier on in the planning stages with a mass timber specialist like G-frame Structures. There is a lot of thought required and by partnering with a specialist early on you get greater cost certainty.’


Harry Reid, Architect at Fletcher Crane Architects, comments: ‘The majority of new buildings are still using traditional construction, but the question is – is it a responsible piece of architecture in terms of materiality and climate? Architects need to have this at the forefront of their minds and persuade stakeholders and clients down this route, even if it ultimately costs them a bit more.’


Fletcher Crane Architects completed Water Gardens Pod in 2019, delivered after winning a competition held by the Church Commissioners to design a home for private rental at Paddington’s famous brutalist Water Gardens Estate. The innovative new home slots neatly between two existing buildings demonstrating the versatility of CLT, and its ability to overcome tough design and site requirements. The mono-pitched pod, which accommodates three inward-facing bedrooms and three bathrooms, features exposed CLT internally and links to the main building by a glass entranceway.



It is clear that airspace extensions offer a new avenue of opportunity for innovative construction. Lightweight engineered timber solutions are ideal for the versatile and flexible design required to make the most of these sites. Its environmental benefits can only add to its suitability as a construction method for achieving airspace extension.


About the author

Lee Murphy, Managing Director, G-frame Structures


This is an extract from the Timber 2021 Industry Yearbook. Download the article with additional images here