Do the adding of air-con, new paint, new door/gate, etc help to enhance the valuation of the HDB flat?

Adding new fixtures like built-in furniture, new coat of pain etc might have a slight increase in the valuation. But it wouldn't be a significant increase of more than $10k as compared to units in your neighbourhood. The key factors in determining the valuation is the location, level of unit, amenities in your area, age of flat etc. Even if you have spent $100k on your reno, you would be lucky if the valuator add on $20k to the average valuation price in your area. That's my view. Cheers!

Another key factor is past resale transactions around è area.