Thousands of garages in Auckland house teens, grandparents, tenants, businesses, games rooms, media rooms, home gyms and more.

For many homeowners converting the "wasted space" of a garage into more usable space makes sense.

To do it legally -- and you'd be crazy to spend that money and not have it above board -- can be tricky, thanks to the hoops owners must jump through with their council.

There are two types of garage conversions. The easiest is where the room just adds space to the existing home, but can't be let as a separate tenancy, says Mark Trafford a project manager who runs Maintain to Profit.

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For this you need just building consent. Bathrooms are allowed if they meet the rules, but you can't put a kitchen and laundry into a basic conversion. However, it is sometimes possible to add a sink to a simple rumpus or art studio conversion without falling into the resource consent regime.

The second kind of garage conversion is a "minor dwelling", which is a self-contained unit with its own kitchen and laundry facilities. A legal attached or detached minor dwelling can be let to tenants, but requires resource consent as well as building consent.

The first type of conversion can be straightforward, says Trafford. The actual building work for a simple garage conversion into warm dry space may only cost $5000 to $10,000, says Trafford.

Older 1970s properties may not be properly lined, which means they need a damp proof course added and additional concrete laid to the slab, says Ian Penniall, director at Harbour City Plans.

Where some owners make a mistake is not realising they need building consent for all conversions from "from non-habitable to habitable space", says Penniall.

The tricky bit in getting that consent is that owners need parking for two cars on the property. That may mean creating external spaces or building a carport.

The building consent will cost around $2000, depending on the council area the property falls into. Consent requirements require that even a simple conversion needs a licenced building practitioner with a design licence such as an architect, which adds cost.

Adding a kitchen and/or laundry requires resource consent as well as building consent, says Penniall.

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Owners sometimes think they can get away with adding a benchtop oven to the bench. However, they could invalidate their home insurance cover by doing this.

A minor dwelling can cost up to $25,000 in plans, consents, and development contributions and connections before a single paintbrush is lifted.

As a result many owners decide it's simply not cost effective to do the full minor dwelling conversion, says Trafford.

But converting the garage to additional living space or a minor dwelling can add value to a home if done well.

Trafford cites the case of a two-bedroom Kohimarama unit, which cost $150,000 to convert into four bedrooms, but added $250,000 to the value of the home.