There are a lot of factors to consider when deciding whether it is cheaper to add on or build up. One important factor is the cost of materials. If the cost of materials is higher for building up, then it will be more expensive to build up. Another important factor is the labor costs. If it costs more to add on, then it will be more expensive to add on. The last factor to consider is the time frame. If you need to add on sooner, then it might be cheaper to build up so that you can get the job done quicker.