HDB plans don't show very much information apart from the location of walls, columns, doors, windows and sanitary fittings. You will only know more of the constraints when you visit your apartment.
There are a few things that I look out for when taking stock of the site prior to designing. The 3 basic things are the location and clearance of the beam, Sanitary Pipes, Floor Traps.
BEAMS - This is important because you need to know whether you can place a full height wardrobe there. Can you image during construction you realise that your part of your wardrobe sit under a beam. You will end up with half your wardrobe full height and the other half ... well you get the picture.
There was one project I did where by we designed everything, about to start construction, and the Client's Feng Sui guy came in and said the bed cannot sit under a beam. Guess what we had to re-layout the master bedroom & bathroom. Thanks for letting us know at the last minute Feng Sui Guy. I've got a bone to pick with these Feng Sui guys but I'll leave my 2 cents worth for another day.
PIPES - Pipes are a bane especially in HDB flats. Technically you can't conceal them. Logic is that they need to be accessible in case there is a choke or leakage. You can box them up but with access panels. So in order to box them up and have it integrated into the design you will need the size and location of these pipes. If not the box up will end up looking like an after thought.
Foor Traps - Its really simple. If you want to do a re-layout of the Kitchen & Bathroom Sink etc... You need to know where the floor trap is. This will help you make a decision if it's possible to link up your sinks to the location of the existing floor trap.