Does renovation add value when you sell your HDB flat?
It really depends on who the buyer is. If the buyer is looking for a renovated unit with move in condition, then your renovation done to it might be of value to them.
Can you sell it at a much higher price to that buyer?
It still depends on the valuation and how much the buyer is willing to pay additional for it. Because, if the buyer has money to pay more for a renovated unit. That means they also have money to renovate the place to their liking. Usually, buyers that look for a unit in move in condition has a tight budget. So selling at a higher price because of the renovation might not work for them.
You really need to find a buyer that appreciates your unit for what it is, with the renovation that you have done for it. The buyer also needs to have similar taste as you. Only then will you be able to sell the unit for the renovation done in it. If not, the new owners will just want to tear everything apart and build it the way they like it with their own taste. This buyer will not see any value in the renovation that you have done to it.