Since you are mainly engaged in family design and construction, and also engaged in material sales, you should let the tax bureau approve two tax rates for you: design and construction, tax rate13%; Sales of goods, the tax rate is 9%.
If you don't provide materials, only design and construction, and you can also provide some auxiliary materials, then you can say that it is a "contractor" or a-supplied project. You can choose to apply the simple tax calculation method to calculate and pay VAT at the rate of 3%, and you can't deduct the input tax, so you need to calculate it separately. Not an old project.
If the signed contract contains the floor provided by your company and other items that need to be integrated with the building through installation and construction before they can be used normally, then you can't choose to apply the simple tax method to calculate and pay the VAT at the rate of 3% in any case, and you can only apply the general tax method to calculate the VAT output tax at the rate of 9%. Because the floor needs to be installed and integrated with the building before it can be used, the applicable tax rate for installing the floor in construction is 9% instead of 13%.
Only when the floor is sold without installation, the tax rate for selling the floor is 13%.