What you need to know about building a house behind house in Perth

So you've been staring at your backyard now for a couple of years and wondering what could I actually build in my backyard. You're wondering how long will it take and how much will it cost! More importantly, you're wondering how much effort do I have to put into this to make it all happen. Building a house behind house (HBH) is easy if you have the right people with the right experience!

You've come to the right place because we specialise exclusively in HBH design, development and construction.

So what is a house behind house!

A HBH is quite simply a house plan that is specifically designed for the rear of your block that sits behind the older existing home. Unlike a street frontage residential style home, there are dozens more requirements needed.

Common terms used by the real-estate and building industry are - “rear strata development”, “rear strata home”, ‘battle-axe development”, “battle-axe house design” or “rear block home design”. Well refer to all of these as a “HBH” for short!

See the diagram below…

Street Front Design versus a House Behind House Design!

Street Front Design Requirements & Differences

House Behind House Design Requirements & Differences

As you can see, there is nearly double the requirements when building a rear block design. Below we will explain in detail the steps, time frames and costs!

But can I build in my backyard!

Before we get into the Nity-gritty, it's important you understand how we work out if your property has the potential to build a second home!

As the population of Perth continues to grow, local councils are lifting the density of allowable homes per lot area. Let's use a real-life example such as the City of Bayswater where a lot of the blocks are around 750 square metres and have a zoning of R25. If you go to the Western Australian planning commission's 100+ page booklet, right near the back is a section called “tables”.

Table 1 of the Residential design codes

If you zoom in to R25 on column 3 you'll see that it says minimum 300 average 350. What this means is the minimum block size allowable to hold two homes (dwellings), must be a minimum of 700 square metres. (350 average x 2). Providing you have a minimum block size of 700m2 for R25, you have yourself a block that can be developed! The "min" states 300. This means the front or rear block does not have to meet the average but can be as small as 300 square meters. This is a bonus given most homes 30 years ago were built smack bang in the middle of the block. When we design the home and block, we often find the rear block relies on the minimum to meet council and state requirements.

If you don't want to work this out yourself we are only a phone call away, and can clearly advise what you can or can't do on your block fairly quickly!

Let's presume now that you do have a block that can be subdivided and has the capability to have two homes.

This is far from the case. If your block meets the WAPC and council requirements, you have the granted authority to build two houses on the One title. It is only by choice or possibly a bank requirement that you would create a second title.

 House behind house steps and checklist

Step 1

Does my block have development potential, (refer R-Codes above - if yes keep reading, if you're unsure send us a message and we will confirm).

Step 2

Initial Block Feasibility - This is where through scaled Arial photography and half a dozen sources of information we can confirm initial feasibility.

We look at a number of items, but what is critical is -

1. Minimum driveway distance between the existing home and the fence

2. Council policy

3. Available space in the backyard

Although most councils will allow a 3-meter minimum driveway width, there are some councils that want up to 6 meters such as the City of Belmont. The client had no choice but to demolish the front home and build two new homes, one at the front and one at the back (Duplex).

Initial Block feasibility

(What we look for when assessing rear block construction potential)

Step 3

Confirming external costs

This is the most daunting and longest digestion phase that our clients have to understand. What you must realise and recognise here, is we can't just pick a house up and drop it in your backyard and walk away. There are engineering, Water corporation, Synergy electrical, gas, NBN requirements that the rear block must meet and proven!

The easiest way to explain this is if you were to go out to the back of Ellenbrook where they have produced thousands of blocks, you would find every single block has infrastructure and services. There is drainage below the road, curbing, lighting, sewer, gas, and an electricity dome all out the front of the block for sale. Whether you are building strata or a battle-axe development you must meet infrastructure requirements.

A feasibility I did this week, came in at $57,000 of Site and development costs to set the block up so it meets the requirements. We will use this template as a good example. This block was in the suburb of Cloverdale in Perth. It was flat and as far as Rear strata developments go, it was fairly straightforward (apart from the shock from the client on the cost). If you want to learn more about the details of each individual cost associated with any house behind house, battle-axe or strata development, please listen to the recorded audio message and I will explain in detail why each cost is essential. Regardless of your budget, it is always best to work backward off the numbers. That is - External costs less your target budget = the amount left for the construction of your home!

Step 4

How big a home can I build in my backyard

Now we know what our site and development costs are we can deduct that off our target budget to work out what's left for the home! But to double-check we need to know how big or small a home will fit into your backyard?

To work this out we need to look at the available buildable area in the backyard. As you can see the approximate available area is 300m2 and the driveway is 80m2. The client selected a built strata development which entails the driveway area is shared. Checking with the council they will allow a 55% footprint on the rear lot. The assessable available area is 340m2 (300+40). 55% of this area is 187m2.

Area of a home that is assessed is-

Garage 35m2

Storage 4m2

Porch 4m2

Internal area of the home 144m2

Total 187m2

An alfresco area will be approx 14m2 and eaves will be approx 15m2, meaning the total roof area will be approximately 216m2. Using a well prices cheap builder, we would anticipate a square meter rate at $780 to $850 per square meter. Working on $800m2 we will probably have construction costs of $172,000.

Show Arial photo of lot with approx area TBA

Step 5

Survey

If Steps 1 to 4 look financially OK! The next step is to now confirm the rear block area exactly and the site and development costs, exactly.

As a guide, a survey will cost approximately $650 to $1200. At this stage, a common reply is I don't want to outlay the money because other builders are giving me a quote and a design without it! That's fine, we can do that as well, BUT you need to be prepared you'll be signing a blank cheque instead of finding out the costs upfront. I've have seen too many times where a client has signed up with a builder at one price, only to cancel everything after receiving their contracts. Don't be a dummy and fall for the easy and fast way out. Do it properly and pay for a survey and you can point the finger back at the builder (Us) if they mess it up! I may sound like a broken record here, but I have had to explain this to people so many times I made a recording "Why Do I Need a Survey!" (listen below)

Step 6

Design

Yes, finally the fun stuff! Now we have your survey back (Takes approx two weeks) we can confirm the build-able lot area, driveway width, plus services locations. Site and development costs can be confirmed. We will now have you complete a design wish list of what you are targeting in your home. We will put together a beautiful home, built to the correct boundary locations and free of any anomalies such as bores or old septic tanks. Example rear lot designs go here.

Our role is to keep working with our design team until we reach a happy result. We may also consult the shire for feedback on setbacks, site coverage or turning circles.

Step 7

Quotation

This is where we have the site costs and house construction costs confirmed. Upon presentation, you would receive a -

• Scale floor plan

• Site plan

• Site costs quote

• House price quote

• Any additional or option costs you may have requested

• Specification