EnerPHit for purpose

Bob and his wife have completed a phased retrofit of tight building into an highly efficient and carefully designed home which meets the needs of the future, both theirs and environmentally speaking


It took all of Bob Prewett’s passion and drive as an eco-build enthusiast to successfully complete his retrofit of a compact 1960s ‘shoebox’ home for his wife and two children in south London, after years of intermittent progress.

The project, in the quiet suburb of Sydenham, London, illustrates how a low energy refurbishment can be effectively executed in stages, even if you may not possess sufficient cash flow upfront at the project’s outset, and while living the property.

Bob and his wife, also an architect, have extended and reconfigured the home to tailor it closely to their family’s requirements, while also upgrading the building to achieve the Passivhaus EnerPHit Standard.

If you’re unfamiliar with EnerPHit, it is very similar to Passivhaus, however while the latter applies to new builds, EnerPHit is designed to provide similarly high levels of energy efficiency when retrofitting an existing property. By applying this standard, Bob’s has dramatically reduced the home’s energy usage.

Bob’s passion for sustainable design stems from studying architecture at the University of Bath, which included learning skills such as heat loss calculations for buildings. In 2005, after completing his studies, Bob set up his own practice to put what he had learned to the test, with, he says, the main goal being “making buildings more energy efficient.”

However, as Bob admits, at this time the majority of people “weren’t interested in building eco-friendly homes.” However, in 2008 a client approached the practice to undertake an “extreme retrofit” on a Victorian terrace house, which he says “shifted our whole focus.” 

This transformative project ignited a desire within Bob to experience the same thermal comfort that he witnessed his clients enjoying. He found himself envious of the cosy atmosphere they had achieved. Bob recalls visiting the home after the main retrofit had been completed during a particularly harsh London winter. There was no heating due to the boiler not yet being commissioned,
but a decorator was comfortably working in short sleeves, using a lamp for warmth. The encounter left a strong impression on Bob, fueling his determination to create his own energy-efficient haven.

A few years later, with a growing family, he finally decided to take the leap. Originally, the plan was to find a site in London and build from the ground up, but Bob admits: “unless we won the lottery, buying even the tiniest parcel of land in central London was a hopeless dream.” Their work commitments meant the idea of commuting was off the table.

So they shifted their focus to finding a “fixer-upper,” a property which they could “radically” alter and make it their own. Bob says this would be the closest thing they could do to building a brand new house.

After an arduous search in a chaotic housing market, they eventually found an ex-council house in Sydenham. Neglected for nearly 60 years, with the exception of some shoddy installations of cheap PVCu double glazing in 2000, the home was in poor condition; the doors and windows were “literally falling out.”

Bob describes it as a “shoebox” with a “mean” layout over two floors with a flat roof. The ground floor contained a kitchen/dining area at the front and rear living space. The lack of natural light left the middle part of this floor dark and gloomy. Four small bedrooms were squeezed onto the upper floor; two being tiny and barely accommodating a single bed, says Bob.

The silver lining of the home’s condition meant that it was on the market for a more affordable price. Despite its cramped interior, Bob and his wife could see themselves living there, albeit with some major alterations, and in 2014 they purchased the property.


Many of Bob’s clients have Victorian houses in conservation areas or even listed buildings, and planning could take years to get, and is “always traumatic,” he says. He was expecting the worst with this project, he says.

A big part of the pessimism stemmed from their intention to add an extra bedroom on the flat roof. “There’s a standard clause that states ‘we don’t support applications that raise the ridge height of a home,’ and we were definitely going to do that!”

Fortunately, as an existing water tank sat on the top of the structure, this provided enough precedent to get planning through without any objections. “For once, it was easy!” Bob exclaims.

Spreading the costs of the project into phases was always going to be a likely scenario: similar to many of Bob’s clients, he had a set budget from the outset, but as is often the case, it fell short of their ambitions. Realising that they didn’t have enough funds, they used a builder’s expertise and together devised a plan to split up the work into three phases. When construction started onsite, the family opted to rent a nearby flat, eventually moving into their unfinished home after the main works were complete.


Phase one took place in 2015, and was compared by Bob to “putting a sweater on the building,” and consisted of most of the energy saving measures. Each wall was injected with cavity wall insulation while further internal insulation was added to some walls. 

Bob made the loft extension one of the first tasks. With this happening, it meant the original uninsulated roof covering which was probably “about to start leaking,” could be renewed. The timber extension required “unusual carpentry work and a careful approach by the contractor.” The detailed design work could only happen after they had opened up the existing roof to see how it was previously made. Consequently, the engineer had to visit the site and quickly sketch detailed plans. 

This phase also saw a high level of ‘strip out’ to change the internal spaces. The reconfiguration of the interior layout was chiefly done to enhance space and functionality for the family. The loft extension added an extra room, while on the ground floor, the separating wall was removed, creating a spacious open-plan area.

To improve the dark central portion, two large rooflights were installed on either side of the loft conversion. Joists on the first floor were cut out in places to allow natural light to permeate down to the ground floor. This significantly brightened the ground floor and transformed it from how it was used previously, says Bob.

Upstairs at the front, the wall separating the small and medium-sized bedrooms upstairs was demolished to create a larger, master bedroom with ample space for clothing storage and a luxurious super king bed. A crucial aspect of this phase involved planning the ductwork for a ventilation system to be installed at a later stage. As Bob was targeting the demanding EnerPHit standard, an MVHR (mechanical ventilation with heat recovery) system was always part of the plan. Bob says a good ventilation system is vital to ensure air quality in energy-efficient homes. 

The challenging part of this stage, he admits, was “probably us, as clients.” Notably, they altered floorplans the day after the builders commenced work, relocating the kitchen to the centre of the house. This decision saw the contractors having to dig up a portion of the concrete slab to accommodate a new drainage run. Fortunately they were working with “great builders” says Bob, with some relief!


By 2018, they had entered phase two of the renovation, with the family already having impressively coped with living in a home being retrofitted for a couple of years. During this stage, they made significant changes to both the front and rear facades by demolishing and reconstructing them using a more insulated wall construction.

Bob explains that their home, like many built during the 1960s, was built using ‘cross wall’ construction. This meant that the party walls bore most of the structural load, and the joists ran from side to side, leaving the front and rear elevations as simple infill.

During the Easter break in 2018, Bob’s wife took the children to France for a couple of weeks. While they were away, the builders managed to knock down the front and rear elevations, rebuild with extra insulation and then fit the triple glazed windows by the second week. By the time the family returned, the house was back together, and fully watertight.

At this stage, the MVHR system had not yet been installed, prompting the family to rely on opening and closing windows. However, as winter set in and the windows remained closed more often, Bob observed an “unsurprising” decline in air quality. He adds: “If you’re building envelope is done to a high standard and you’ve not commissioned the ventilation system yet, it can be a big problem.”


The final phase of the project has dragged on for a couple of years, admits Bob, with “quite a bit of work still yet to be finished.” This phase has seen the final major fabric upgrade however, where the outdated PVCu doors at the rear were replaced with a larger set of sliding triple glazed doors, providing a view of the newly decked back garden.

While the upstairs finishes are still awaiting completion, considerable progress has been made downstairs. The area has been adorned with customised built in timber furniture that utilises the small apertures within the home, effectively maximising the available space. 

The finishes are now mostly being undertaken by Bob, who has turned it into a weekend activity. Consequently, the project has been somewhat prolonged, but his commitment and efforts are driving it forward steadily. 


The home as it stands today displays various timber details as well as white finishes as part of its clean and simple contemporary design. Bob asserts how timber was used to “reflect the values of building an eco home.” 

As you come in through the front door, where there used to be a small WC is now a compact shower room. To the left is a functional space featuring a bench where the family can take their shoes on and off, as well as a decent amount of storage. “It’s very small, but it just about copes with the clutter that a small family can create,” says Bob. 

Continuing past this area, the dining area is on the left, flowing seamlessly into the central kitchen. A corridor then brings you to the modestly sized living room at the rear of the home. “It’s a very simple, open plan layout, with dining at the front, kitchen in the middle, and then relaxing at the back,” asserts Bob.

Moving up to the first floor, there remain similarly-sized bedrooms at the rear of the house, maintaining the original proportions. But in the middle of this floor is now the family bathroom, while a generously sized bedroom sits at the front. Ascending stairs to the second floor, there is a compact fourth bedroom nestled within the attic space.

Part of the home’s charm lies in its raw elements, with exposed joists replacing ceilings on the ground floor. Not only does this “give you a feeling of a bit more space above your head,” says Bob, but “it also allowed us to do some of the airtightness measures!”

However, one of “the most important” parts of the home for Bob is how natural light flows through the ground floor space. Bob makes the most of this working on a bespoke timber desk here when working from home. This desk cleverly doubles as a cupboard, opening up during the day and neatly folding away its
doors at night. “It’s such a nice place to work,” claims Bob.

Undertaking a phased retrofit project has presented both advantages and disadvantages for Bob and his family. One of the main benefits, as Bob points out, is the flexibility of not requiring all the funds upfront. However, he also acknowledges that completing the project all at once under a single contract would likely be more cost-effective. 

Another benefit has been how splitting the project into phases has allowed them ample time to carefully consider each decision, avoiding rushed choices that can lead to mistakes, which he has seen with his clients. ”The extended period gave us a chance to see how the project is unfolding, giving us the opportunity to make better decisions.”

However, living in an unfinished home for an extended period has been stressful, as Bob admits. The constant reminder of incompleteness has kept them on edge during the journey.

Despite the challenges, Bob and his family have found great satisfaction in the results achieved. Bob confidently admits that his home now surpasses the comfort level he once admired in his client’s home all those years ago! Is it now EnerPHit certified officially? – yes it is.