House extensions: Everything you need to know

Adding a basic bedroom home extension can cost roughly $60,000 to $120,000, but multi-room extensions and double story extensions can cost much more. Anywhere from $120,000 to $400,000+, according to Add Value Renovations. 

• Basic bedroom: $50,000 to $100,000+

• Multi room extension: $100,000 to $250,000+

• Bathroom extensions: $50,000 to 120,000+

• Garage extension: $75,000 to $120,00+

• Double story extensions: $200,000 to $400,000+

The above figures should only be used as a very rough guide. Your extension could cost much more, or much less depending on the size, materials used, the site and your location. 

Above a certain minimum, you decide how much your house extension will cost. Here are the four main factors that will determine the total amount you spend:

Larger, more complex extensions obviously cost more than simple, small additions, and multi-story and multi-room extensions can be extremely expensive. When you extend your home it’s important to make sure you only build what you need. 

The cladding, internal fixtures, fittings and finishes of your extension can greatly affect its cost. Your contract with your builder may also include variable costs, which can change to cover your choice in materials (or changes in prices). It’s a good idea to choose as many materials as possible before the build starts to provide more cost certainty. 

Sloping sites with unstable ground can be much more expensive to build on. If your site needs work it’ll increase the cost of design and building – plus you’ll need to pay for earth works and specialist geotechnical consultants. It’s always best to know the quality of your land early in the process to avoid cost blowouts. 

If you’re extending your home, you’ll almost always need to apply for council consents and code of compliance. The cost and complexity of these will depend on the type of changes you’re making, but they can add up very quickly (and take months to process). More on this later.

Rough planning

First thing’s first. Before you make any calls it’s a good idea to have a think about what you really need. Is it a small extra office? A separate bedroom, bathroom and kitchenette for guests and Airbnb? Or a multi-room multi-story extension? 

Think about whether your family will grow in future and how much you’re likely to use any extension. 

Choosing an architect 

Once you have a rough idea of what you want, it’s time to choose an architect. Go for someone local with a good reputation, lots of experience and plenty of positive reviews online (or take a recommendation from a friend). It’s best to go for someone who’s designed similar extensions before if possible. 

Initial consultation and feasibility study

Once you’ve chosen an architect, they’ll have a chat to understand what you want, visit your site and present initial design concepts. They should also conduct a brief feasibility study and let you know roughly how much your extension may cost. 

Feasibility will include checking with local council rules to see if you can build what you want. 

Finance

As soon as you have a rough idea of what your house extension will cost it’s a good idea to secure finance. Most Kiwi who build house extensions in NZ pay for them by extending their home loan. If you’ve got equity in your home you may be able to do this too – speak to a mortgage broker to find out more. 

When financing it’s always a good idea to get 10-15% more than you think you may need, to cover cost blowouts. 

Development of house extension design and submission with council

Next, your architect will develop a final draft of your house extension design with your feedback in mind and submit this with your local council for initial building consent. You’ll have to wait for approval from the council before you can start building. 

The final design should include specification of materials, finishes, layouts, fixtures and fittings. At this stage, it's important to communicate openly with your architect to make any changes early, before plans are submitted to council for approval. 

Choosing a builder 

As always, check reviews before choosing a builder or renovation company and feel free to speak to several to find one you like.  You’ve got two options here – you can either:

1. Manage the build and all contractors yourself or,

2. Hire a builder or project manager who will manage the entire project for you. 

You may be able to save costs and have more oversight with option one, but it’ll require lots of work and coordination. Most people are better off hiring a project manager or renovation company to do the work for them. 

Get quotes and finalise costs

Once you’ve spoken to a few builders and/or renovation companies, it’s time to get down to brass tacks. It’s a good idea to get a few quotes for the work to make sure you’re not overpaying. 

Keep a close eye on variable or PC sums in the quote. These are prices that aren’t fixed, which means they can change down the track and add significantly to your project's cost. It’s always a good idea to minimise these if possible. 

It's common courtesy to let your neighbours know to expect construction noise before starting extensions, and in some cases your consent may even require that you notify them (or in some cases get their permission). Let them know what to expect, how long the construction will go on, and provide your contact details in case they need to get in touch or ask questions. 

Every year, hundreds of renovations and extensions are held up because of council consent. Don’t make the same mistake. Speak to your architect and/or building company about applying for the relevant consents early, and make sure you know exactly what’s required to avoid costly delays. 

A solid contract can spell out costs and timelines clearly so that your building company has clear goals to work to. With that said, most projects do tend to take longer and cost more than initially intended. Don’t let this catch you by surprise – leave space in your plans for extra costs of at least 15% and later completion.