The first step is to select an architect and a builder – preferably ones who have worked together previously. You can waste a lot of time and money if the builder doesn’t understand the architect’s plans or directions.
Your architect can advise on different builders so you can obtain quotes. If you’re renovating so you can sell the house, paying a professional builder for a better than average finish may net you many times what you’ve spent when you do sell. On the other hand, you’ll either be living there during the renovations, or paying to live someplace else, so you need to budget and decide what you can afford and tolerate in terms of living conditions.
Your architect should be able to assist you with council to ensure your plans are compliant. You may also need to meet with the neighbours who will be impacted by the construction and finished building, to overcome any potential complaints or issues they have with your plans. It’s always better to have as much knowledge about potential problems before you submit your plans, as revisions can be costly and time consuming.
Are you going to extend upward, outward, or downward? Make sure whatever renovation you choose complements the original structure. Costs will vary based on the features of the site, but in general terms, it’s usually cheaper to extend outward, rather than up or down. Going out rather than up leaves your roof line intact. By contrast, going up means the roof has to be removed, the lower storey weather protected, and then the roof re-married to the house.
A single-storey addition is all gain. But going up has its advantages as well – chiefly that it leaves your garden intact. A second storey will also help insulate your first level. You avoid site costs such as footings and slabs.
Don’t build up for a single room, especially if you need extra space. The space gain will be impacted by having to add a staircase. You’ll want at least two bedrooms, and maybe a sitting room or en-suite from your new upstairs space.
Excavating is another option, but it’s nearly always the most expensive, particularly if your home is on a slope. Generally an underground extension is for a garage, wine cellar, family room, or perhaps an office. It’s usually not suitable for a bedroom, unless you can build a self-contained apartment. You’ll need to consider what you’re digging into as it costs more to excavate rock than soil or sand, although it usually doesn’t require retaining walls.
Try to make as few changes to the existing structure as possible. Be mindful of your existing property’s aesthetics. Try to match the most prominent features, such as the roof, as closely as possible. Use similar building materials wherever possible. When adding new doors and windows, line them up with existing fixtures.
It also usually pays to let your architect project manage the job. They understand builders, council requirements and your budget, so usually pay for themselves in the hassle and money they save you.