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How Delve Architects united Miami and Ibiza at sustainable Hobbs House

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Andrew Pearson

A smart approach to planning was key to winning consent for Delve Architects’ reconfiguration and highly sustainable retrofit of a suburban home to meee their client's three-way brief

Hobbs House is the £650k (plus VAT) transformation of a 1930s semi in Winchmore Hill into a contemporary energy-efficient home.

How did Delve Architects first become involved with this project?

This was Delve Architects' first project to come through social media. Our client, Jane Hobbs, had been following us on Instagram knowing she was planning to do a domestic renovation in North London. Eventually, she messaged via instagram to ask if we would we be interested.

She was living in Winchmore Hill and wanted to stay local, but needed more space. She'd found a new home but hadn't bought it yet, so we joined her on the viewing to give some advice as to what, potentially, would be possible.

The house is semi-detached, on a corner plot facing southeast. It was built in the 1930s and included a mix of different spaces and small extensions. You could see it was not working hard enough: access to the garden was poor; the rooms were small and the garage had ended up in the middle, blocking the flow. We knew it could be extended to the side, to the rear and in the loft to create a new second floor level, so you could see where volume could be added.

Credit: Chris Snook

What was your brief?

The initial brief was a challenge. Externally, the client wanted a Miami Art Deco box with black Crittall-style windows and white render. Internally, she wanted a soft, bohemian, Ibizan beach feel. At the same time, we were asked to make it as sustainable and eco-friendly as possible. So we had these two juxtaposed styles plus the eco-credentials to merge.

How did the planners react to the proposal to radically remodel the exterior?

The hardest part of this project was obtaining planning. Initially, we went for something quite provocative: a true flat-roofed Miami Art Deco design. It was refused. We appealed. The appeal was refused. We resubmitted with something smaller. The same process happened. In the end we decided on a hybrid approach of full planning and permitted development rights.

This meant we had to split the project into different packages: a loft conversion, a ground floor extension that wrapped around the side and rear of the home, and a first floor extension. In this way, we got them all. We also used the existing footprint of the garage, which would be knocked down and rebuilt as part of the house.

Credit: Chris Snook
Credit: Chris Snook

What sustainability features did you consider?

Once we'd developed the design to the point where we were ready to submit it for planning we explored ways to increase its sustainability. We looked into whether SIPs or CLT could be used to build the extensions, but the project was too small for these to be viable. Instead, the ground floor extension has dense thermal block cavity walls, while everything above is built in timber frame with insulation between the timber studs and on the outside.

We added as much thermal insulation as possible to the floors, walls and roof. On the internal face of the external walls, including the existing walls, we've used 32.5mm foil-backed insulated plasterboard. By continuing the insulated plasterboard into the existing parts of the house we were able to improve its thermal performance and achieve a clean finish.

How did you decide on the most suitable eco-technologies?

Jane wanted to use PV panels and solar tiles so as to collect as much solar energy as possible. She also wanted to move away from gas. We asked [energy consultant] Mesh Energy to put together a renewable energy feasibility study. Mesh made it clear that with the home's form and orientation it was never going to generate a surplus of electricity, even with a storage battery.

We did ask about using solar thermal, but there was insufficient area on the south-facing front roof to accommodate this and two new Velux windows.

The client was advised that solar PV tiles, which are a relatively new technology, was less efficient than big panels. Jane did not like the appearance of the big panels, so to protect the appearance of the front of the house we've used PV tiles here. On the flat dormer roof at the rear we've put as many large panels as we could because they are hidden from view.

  • Credit: Chris Snook
  • Credit: Chris Snook
  • Credit: Chris Snook
  • Credit: Chris Snook
  • Credit: Chris Snook

How is the house heated and ventilated?

We have successfully moved away from gas heating to an electric air source heat pump (ASHP). The house has quite a large garden, so we did consider a ground source heat pump. However, the available garden area and the depth required to install the collection coil meant that an ASHP was much more suitable.

The heat pump supplies the underfloor heating, which is installed throughout. It's an incredibly efficient system; each room is controlled by thermostat, with the temperature set by the owner using a phone App. She also wanted a wood burning stove, but was advised by Mesh that because we were putting in so much thermal insulation she would be far too hot.

Fresh air is efficientlyprovided throughout by a mechanical ventilation with heat recovery (MVHR) unit. We had made a big decision early on to demolish the existing staircase and relocate it to the other side of the building. This formed a vertical core up through the house and allowed us to create service voids on both sides of the stairwell to accommodate the ventilation ductwork and other services.

  • Credit: Chris Snook
  • Credit: Chris Snook
  • Credit: Chris Snook
  • Credit: Chris Snook
  • Credit: Chris Snook
  • Credit: Chris Snook
  • Credit: Chris Snook

Where is all the plant accommodated?

A big challenge was the space needed for plant. In addition the MVHR unit, there is the PV storage battery and all its accompanying electrics, electrics for the heat pump, an electrical distribution board, and we've also got a 300 litre Megaflo unvented hot water cylinder, so the only place we could put it all was in a room in the loft.

The problem with this was acoustics because there are bedrooms underneath. So we used a suspended ceiling filled with Rockwool acoustic insulation and acoustic plasterboard, and wrapped as much of the ducting as possible in Rockwool to help mitigate any noise.

Outside we have also installed rainwater harvesting. Rainwater is stored in a large tank in the garden, hidden beneath the greenhouse. In addition, wherever we have a flat roof, it is covered in sedum to help increase biodiversity and improve rainwater retention. The soil and plant matter also adds additional thermal insulation to the roof build-up. This helps keep the spaces cooler in summer and warmer in winter, while the carpet of sedum hides the single-ply roof membrane from view [see detail].

Similarly, the front garden incorporates a porous car parking space to help limit rainwater run-off and to encourage biodiversity.

Now that the project is finished, is your client still following you on Instagram?

Yes, she's still following us @delvearchitects. Jane loved all the green discussions but her priority was always to have a really attractive house to live in. Our challenge with the interiors was to hide these technologies away so that Jane gets a timeless property to live in and enjoy while in the background the MVHR provides her with constant fresh air, she's generating solar electricity, she's not using any gas and she's collecting rainwater. She is thrilled with the home's transformation.




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