We need to build more homes. This is not optional. And they need to go somewhere – ideally where the life-cycle emissions of the whole city will be lowest, writes Eleanor West of City for People. 

Right now in Wellington, “special character areas” cover 88 percent of land parcels within walking distance of the city centre. These zones preserve old wooden villas and prevent us from building lots more homes at higher densities in the best parts of the city where heaps of people want to live. 

The council has proposed shrinking these “character” zones to focus on protecting smaller sections of well-maintained villas, which would allow some of the older, poor-quality homes to be replaced with new homes. After all, we have a severe housing crisis and we desperately need to make space to build. Putting roofs over heads should be a priority. 

But the independent hearings panel advising the council wants to retain character protections across vast swaths of the inner city suburbs – advice that’s been widely panned. The panel supported several questionable justifications for this position, including that demolishing and replacing old wooden villas with new homes will result in higher carbon emissions. 

It may seem counter-intuitive, but the opposite is probably true.

Historic Places Wellington told the panel it was better to keep old homes and retrofit them to be more energy efficient than to demolish and replace them, if you consider life-cycle carbon emissions – the new frontier of emissions analysis. The core of their claim is that the carbon saved by replacing a draughty old villa with a new, energy efficient home is outweighed by the wasted carbon “embodied” in constructing a new home. 

New homes built with modern technology are more energy efficient, which means lower operational, “in-use” carbon emissions than old homes. But Historic Places Wellington argued you must also consider the embodied, “up-front” emissions – all the carbon (and energy) that goes into constructing a building before you can use it. Concrete and glass aren’t free. These materials cost us to produce and transport. The associated emissions are locked-in before you can even use the building, and they’re wasted if you demolish a building too early, before its useful life-cycle is up. 

So their argument goes: even if you replace an existing home with something modern and energy efficient (much lower operational carbon), the operational carbon savings don’t out-weigh the wasted embodied carbon of the demolished house and the construction of the new house. So, with a life-cycle view, demolishing existing homes increases emissions. It’s a fair argument, and probably true. 

But life-cycle analysis is absolute rubbish if your assumptions are bad. It’s the age-old adage: Shit in, shit out. Their argument assumes we’re trading like-for-like: one existing home replaced by one new home. But this is not the case. We want to replace each home with many new homes. 

Houses don’t exist alone in space. They’re part of a complex system: the city. Comprehensive life-cycle analysis should take this context into account. Wellington has a housing crisis caused by high demand and a persistent under-supply of new homes. We need to build more homes (this is not optional) and they need to go somewhere – ideally somewhere where the life-cycle emissions of the whole city will be lowest. 

We have options. We could knock down houses and rebuild at higher densities within existing suburbs. Or, we could build new houses at the city fringe – this is the counterfactual and we can’t just ignore it. We have to assume that the embodied carbon-cost of new homes will be paid somewhere, regardless of whether or not we demolish character homes. 

We can reduce the carbon cost of new homes by building at higher densities. “Gentle density”, like terraces and walk-up apartments, reduces the carbon emissions per house by using resources more efficiently. Smaller homes that share walls and roofs require less material input (embodied carbon) and are more energy efficient (operational carbon). The higher the density, the lower the carbon cost – to a point. The concrete and steel input required to keep skyscrapers upright gets too high, so there is a sweet spot. 

Gentle density is generally better, but couldn’t we just built townhouses and walkup apartments in new greenfields on the city fringe? Why do we need to build like this in the existing inner suburbs? 

Because density affects how efficiently we use infrastructure. When homes are closer together, it is cheaper – both in terms of the carbon-cost, and money – to service them with infrastructure. Which means the embodied carbon cost of infrastructure tends to be lower per person in large, high density cities. 

Density also helps reduce operational carbon emissions from transport. When people live closer together, it’s easier to connect them to trains, bus routes, and cycle paths, which reduces their reliance on cars. Again, the denser the better: you need lots of people in close proximity to ensure there are enough potential riders to make a light rail worth building. 

Planners have to trade these transport emissions savings off against the density sweet spot for buildings’ emissions. They have to consider the carbon cost of all the other infrastructure – three waters, electricity lines, gas networks, amd more. They have to model (or at the very least, think hard about) all of these issues when making zoning decisions to find the perfect balance: the “goldilocks density”. 

In theory, demolishing and rebuilding at higher densities results in fewer emissions, if you consider the life-cycle of the whole system, accounting for infrastructure efficiencies and reduced operational emissions from transport. But it’s been rare to see anyone attempt to definitively find this “goldilocks density” by considering the emissions trade-off between transport and buildings in the same model. 

This recent Build Up report from the UK Green Alliance makes a good attempt. It compared two scenarios: a “demolish and densify” scenario where 50 homes were replaced with 300 flats, and a “no demolition” scenario where the additional 250 homes were built at a lower density at the city fringe. They modelled the embodied and operational carbon of the homes, as well as the operational carbon of transport, and found the “demolish and densify” scenario resulted in significantly lower carbon emissions over time (even if you assume a switch to electric vehicles), while reducing sprawl into green areas. 

Now, I know the independent hearings panel has expressed some anxieties about accepting evidence without having the opportunity to rigorously question the authors on its relevance to Wellington specifically. But since they seemed prepared to accept the UK-based evidence provided by Historic Places Wellington, I can only assume that they will find the Build Up report equally relevant. It’s a simplified case, but it makes the point: demolishing character homes and rebuilding at higher densities would be better for the environment. The maths will, of course, always be situation dependent – in different contexts, there are different variables that need to be considered – which is why a theory-based rule of thumb will probably continue to be the best we’ve got. 

At the end of the day, I agree that character suburbs are nice. And if there were no consequences to preserving them in their entirety, I’d be all for it! But there are consequences, including higher carbon emissions. I am not saying that we have to demolish all the character homes. But some of them? Yeah. We need the option. Wellington is not a museum, it’s a growing, vibrant, ever-changing city. Let’s treat it right and make sure the “special character areas” are a sensible size in the new District Plan. 

Eleanor West is a volunteer for City for People, a coalition of individuals and organisations working together to advocate for affordable housing in Wellington. She holds a degree in environmental science.