To make sure I understand your question, you're saying that because there's a GST rate reduction of 1%, the builder may decide to increase the price of the house by half a percentage point to benefit from a tax rebate. Is that what you're...?
There's no way of ensuring that—market forces and so on. This can happen for cars, clothes, and for all types of goods. One would expect that a big chunk of the tax reduction will end up in the pockets of consumers. It will also benefit the producers in some instances, but it's impossible to know in advance.