There is no customs duty on furniture, but there is VAT. The goods must be cleared for free circulation on the border if you buy in excess of the value limit.
If you buy goods with a total value of less than the value limit, you are not liable to pay value added tax or other taxes. The value limits are NOK 3,000 or 6,000, depending on how long you have been outside of Norway, and they apply when you travel yourself with the goods.
If you have purchased goods with a total value in excess of the value limit you must pay value added tax. The rate is 25 per cent, and the calculation basis is the customs value with the addition of other taxes. In most cases, this will represent the purchase price and shipping costs.