In most cases, building out (extending the ground floor) is cheaper than building up (adding a second storey). A ground-floor extension typically involves fewer structural complexities, as it doesn’t require reinforcing the existing foundation or modifying the roof. It also avoids the need for staircases, which take up valuable internal space.
Building up can be more expensive because it requires additional engineering work, stronger foundations, and potential roof restructuring. However, it may be the better option if you have a small block or strict land coverage regulations. Additionally, a second-storey extension can add significant value by increasing the number of bedrooms and improving views. The best choice depends on your budget, land size, and long-term goals, so consulting an experienced builder can help determine the most cost-effective approach for your home.