I'm currently rebuilding my house so I do have some advice for you.
You first need to either get an architech, or a all-in-one (AIO) designer-builder that do both design and building to get started on the building plan.
Look and ask around to get reliable architech / AIO builder with style that you like (and budget that suits you). Different architechs and AIO builders have different architerural styles so ask for their portfolio to have a feel.
How high you can build up to depends on the location - It ranges from 2.5 all the way to 4.5 storeys for residential homes. I believe the plot ratio for your address can be found in the URA masterplan website https://www.ura.gov.sg/maps/?service=MP . However, plot ratio is only a guide to the max height that you can build up to and does not set the limit of your buildup area - Your architect is probably the best person to advise you on that.
If rebuilding, you'll also need to do prep work like soil study to determine whether you need piling work to be done for your building (esp in coastal/ soft soil area) and also specialist inspection of the existing sewerage system to determine their existing condition.
After your architech is done with the design (with your input and approval), the plan will got to the architect's professional engineer (PE) for the structural calculatio, design & drawing.
Once that's done, the architect will ask for an open tender(with all the required building specifications listed in the tender document) to select your builder (this step skipped if you're using an AIO builder).
Invited builders who're interested in participating will submit their bid and portfolio. Select your builder based on their price, track record, workmanship, feedback from previous owners etc and willingness to negotiate. I strongly suggest that you have at least 3-4 buildier participating in the bidding exercise and make sure you have at least 1-2 who're not recommended by your architect (in case your architect recommends someone who's paying him a kickback).
Don't be afraid to negotiate for a better price with the builder and make sure everything is written down on the building contract before signing cos after that, anything will be considered a variation order (VO) and that's when you'll face the harsh reality of escalating costs
Building cost ranges from 250 to 300 psf on the average depending on the finish that your choose and may cost up to 350 psf for higher end finishing. Some items will add significant costs - Lifts (including lift shaft) adds another 80-100k, piling will add 100k+ and basement will add another 150+k.
If you can afford it and have elderly at home, I would highly recommend a home lift. Basement usually not recommended due to the extra cost and problem with moisture/water leakage - It's cheaper and better to build 3.5 storeys rather than 2.5 storeys plus basement (unless height limit is an issue and you want as much build-in area as possible, and don't mind the cost and risks of basement).
Hope this is enough to get you started on your journey of building your dream home!
