Appeal 3341078

The appeal is allowed and the planning permission reference 23/02957/HFUL for a single storey rear extension at [site address] , Cambridgeshire granted on 27 September 2023 by Cambridge City Council, is varied by deleting condition 3 and substituting for it the following

Notwithstanding the approved plans, the flat roof of the extensions hereby approved shall be green biodiverse roofs. The green biodiverse roofs shall be constructed and used in accordance with the details outlined below: a) Planted / seeded with a predominant mix of wildflowers which shall contain no more than a maximum of 25% sedum planted on a sub-base being no less than 80 millimetres thick. Appeal Decision: APP/Q0505/W/24/3341078 b) With suitable access for maintenance. c) Not used as an amenity or sitting out space and only used for essential maintenance, repair or escape in case of emergency. The green biodiverse roof to the larger extension shall be implemented before the end of the first full planting season following this decision; and the green roof to the smaller extension shall be implemented in full before the end of the first full planting season following the first use of it. The roofs shall be maintained at all times in accordance with the Green Roof Organisation's (GRO) Green Roof Code (2021) or successor documents.

The main issue is whether the proposal makes appropriate provision for water management and the creation of habitats and valuable areas for biodiversity

Despite the description of development, the council granted permission for two single storey rear extensions. The application form described the proposed roofs as having a single-ply membrane. The condition imposed by the council required that the roofs be green biodiverse roofs in order to satisfy policy 31 of the Cambridge Local Plan 2018. Of relevance the policy advises that development will be permitted provided that: (a) surface water is managed close to its source and on the surface where reasonably practicable to do so; and (f) any flat roof is a green or brown roof.

The larger of the two extensions has already been built although I understand its structure is such that it can be retrofitted with a green roof. The smaller extension is not yet in place. As the works are not complete, I have considered the development as a proposal based on the plans rather than with regard to the works already undertaken.

The supporting text to the policy advises that ‘Green and brown roofs are a key measure in terms of Cambridge’s climate change adaptation policy. They offer multiple benefits for a comparatively small additional construction cost, including forming part of an effective sustainable drainage solution, reducing the amounts of storm water run-off and attenuating peak flow rates.’ It goes on to set out that ‘Living roofs can reduce the negative effects of climate change, for example by improving a building’s energy balance and reducing carbon emissions. The use of vegetation on a roof surface ameliorates the negative thermal effects of conventional roof surfaces through the cooling effect of evapotranspiration, which can also help ameliorate the urban heat island effect (UHI). It can also provide benefit in the form of insulation, helping to reduce the internal cooling load of buildings, thereby reducing energy use and associated carbon emissions. The biodiversity benefits of green roofs are manifold, supporting rare and interesting types of plant, which in turn can host a variety of rare and interesting fauna.’.

The development plan is clear that flat roofs, as proposed, should be green or brown roofs. The proposed details are therefore at odds with the development plan. Planning law requires that applications for planning permission be 2 Appeal Decision: APP/Q0505/W/24/3341078 determined in accordance with the development plan, unless material considerations indicate otherwise.

The appellant has put forward a number of considerations to suggest that such roofs would not be necessary or reasonable in the particular circumstances relating to this property. It is suggested that the extensions would be built over the former site of a large concrete patio so there would be no loss of wildlife habitat or biodiversity. The house is in a rural area and has a very large and mature garden. Measures are already underway to increase biodiversity by adding native hedgerows, a large wildflower meadow area and wildlife corridors of longer grass and wildflowers. The surface water from the rear of the property has already been modified to drain to soakaways and the extensions would follow suit, so already relying on sustainable drainage systems and avoiding pressure on other drainage provisions. It is suggested that the policy makes more sense in urban areas.

The provision of two small green roofs would contribute to surface water management as they would introduce an additional element to the drainage system and slow the rate of discharge from the roofs to the soakaway. No details of the soakaways have been provided but it seems likely that the overall benefits of these green roofs, in these circumstances, would be quite limited, but marginally positive. The scale of the roofs would also limit the scale of vegetation that would be achieved and the corresponding contribution to overall biodiversity. These benefits would therefore be limited and given the nature of the existing garden and the positive actions taken by the appellant, separately from this proposal, these limited benefits would perhaps be even less significant. However, although limited, they remain positive compared to the preferred roof structure.

Being in a more rural area, it is likely that there would be little benefit compared to the thermal effects of conventional roof surfaces with regard to ameliorating urban heat islands. No information has been provided with regard to the relative benefits with regard to insulation, helping to reduce the internal cooling load of buildings and thereby reducing energy use and associated carbon emissions.

As described by the appellant, this is a blanket policy that does not take into account the different environments within the district. It does seem likely that the benefits of small green roofs would differ depending on the particular circumstances of each case. The benefits, in these particular circumstances, would perhaps be at the lowest end of the spectrum. However, given that the policy does not seek to distinguish between such areas and as the proposal would bring some, albeit limited benefits, the matters put forward do not represent material considerations that are sufficient to suggest that a different approach, contrary to the policy should be accepted. To do so would undermine the implementation of the policy more generally.

[name] acknowledge the appellant’s view that the policy outcomes with regard to the treatment of pitched roof extensions and flat roof extensions differ significantly, the purpose of the policy is to provide benefits where these can be achieved and as such, this does not undermine the legitimacy of the policy. 3 Appeal Decision: APP/Q0505/W/24/3341078

Overall, the matters put forward by the appellant do not suggest that a decision contrary to the development plan should be reached. With regard to the condition, Planning Practice Guidance acknowledges that depending on the case, it may be possible for the local planning authority to impose a condition making a minor modification to the development permitted. I am satisfied that this condition results in only a minor modification. Without the condition the application should be refused and as such, it was reasonable for the council to attach it to the decision.

Given that the works have commenced however, the condition cannot now be satisfied as it requires that the green biodiverse roof be implemented in full prior to the use of the extension. It is necessary therefore to amend the condition to ensure that it can be satisfied. I therefore allow the appeal and impose a new condition to replace it.

I have required that the green roof of the already built extension be implemented during the next full planting season following this decision; and the smaller extension to have its green roof implemented before the end of the first full planting season following the first use of it. I have amended the condition to remove extraneous information and make reference to the plural of extensions to reflect the details of the plans. The remaining conditions remain in force although the development has already commenced.