The Science of Sub-Division

Sub-dividing is quite fashionable in Wellington. There isn’t a huge amount of space in the central city for new developments, and so dividing existing units provides a solution to lack of housing. Much of the eastern and northern suburbs are terraced into hillsides, making the undersides of the houses are very viable for creating new dwellings. Further out, areas like Upper Hutt and Wainuiomata have large, flat sections where it is possible to build a new, stand-alone dwelling. Everyone in Wellington knows someone who is ‘thinking about sub-diving at some stage’. 

A sub-division can be a great investment, but it is not a straight-forward process and there are several things to think about before jumping in. In 2022 SGB undertook a complicated sub-division in Newlands, which provides a good case study for anyone thinking of going down this road. 

A new dwelling means more people in an area. The Territorial Authority (the fancy term for the local council) is very interested in this, because more people mean more parking requirements, more electricity drawn in that area and more use of the communal areas such as access routes and outdoor areas. At the same time, a new dwelling means more roof area and therefore more water diverted into gutters and then storm water. More sewage added to the system. More noise. More urban densification. A higher fire risk. 

If every property in Wellington was sub-divided over night, we would have a radically different city. The Territorial Authority is weary of this, and so planning a sub-division all must be done in accordance with the Resource Management Act, and more to the point, the specifics of the Act that different TAs choose to focus on more than others. 

So, what does this all look like in real terms, and how does it affect your potential subdivision? Here is what happened during our Newlands project. 

The existing building consisted of three dwellings – each somewhere between a large flat and a small house. Two are on the second story, and one on the ground, along with what was a very large carport area. The carport area was to be converted into a fourth dwelling on the ground level. 

Here's your first major consideration. Where are your service going to run? In our case all three houses had their mains power, fibre and Chorus running above ground through the carport – and more awkwardly, their egress water (sewage and storm water) coming through here as well. Egress water must be on a downhill slope, and so to achieve this we were looking at some serious excavation to put these services underground. To achieve this, we put the whole upper level on acro-props (temporary supports) while we incrementally poured new concrete foundations. This meant we then installed new steel beams, which were used to both support and straighten the joists above.

This speaks to another major consideration when planning a sub-division. Triggering the need to bring existing structures up to Building Code. The Building Code is the rules that a dwelling must be built by, to make it fit to occupy. A good example is that stairs must always follow specified rises and runs. You can’t build a ridiculously steep staircase, for obvious reasons. The reality is though, most old houses aren’t compliant with modern Code, for multitudes of reasons – bracing units, insulation, tiled splashbacks - the list is endless. When sub-dividing, you will probably have to work backwards into the existing dwelling, and in doing so, you may need to bring it up to Code. For instance, during this subdivision, SGB detached an external staircase to one of the upper flats for access reasons. A Code-Compliant builder can’t just whack the staircase back on the way they found it, they must rebuild it to Code. By building a new flat we also triggered the need to update the access routes to all four flats to be compliant with clause D1. 

If, like us, your sub-division is attached to another dwelling, then fire safety becomes a key consideration, Clauses C1 – C6 of the Building Code. You must create a ‘fire cell’ – a unit which will contain fire and prevent it’s spread through neighbouring units. This is done through use of specified products such as Gib Fyreline, fibre cement board, intumescent sealants and internal fire doors. These products have specific manufacturer’s guidelines for installation, and the council will fail the inspection if they are not done correctly. If you’re planning a sub-division, you need to go through the route of getting consented building plans and then have a Licensed Building Practitioner follow them to the T to be fire compliant. Whatever saving you may have made having your mate’s uncle run the job will be lost when the council makes you redo your fire cell. 

Finally, to get a Resource Consent, the TA will likely stipulate what needs to be done to integrate the sub-division into its surroundings. In our case, this did create an unexpected landscaping package. The council wanted more outdoor communal space built in, to counter-balance the now denser housing. This included things like new areas for rubbish bins, planter boxes and new clotheslines. From SGB’s perspective, it made for a nice final touch to the project, but it was an unforeseen cost to the homeowner, and it is the sort of thing worth being aware of, if considering a sub-division of your own.