first, confirm if your house is a 2.5 storey house or a 3 storey house. some layman would think that a 2.5 storey house is a 3 storey house as they look similar from outside but are classified differently.
second, check if your house is in what kind of zone. easiest way is to look at the neighbours house to see how many storeys the highest one you see have. the more accurate method is to check URA's landed housing master plan.
Assuming that your house is actually a 2.5 storey house is a 2 storey landed housing zone, then you can't built your house upwards anymore. but if your existing house has not been built to it's allowed boundaries, then you could extend your house at the rear and maybe the front (depending on existing house footprint).
if your house is a 3 storey house in a 3 storey landed housing zone, then you can add on another attic storey to the house. BUT this is depending on whether the existing house's total height as URA's envelope control only allows a maximum of 15.5m building height for 3 storey landed houses which is lower than the previous 17.7m height for 3 storey houses. so if your existing house already is near or exceed 15.5m, it is unlikely you will be able to add another attic storey above it.
if you had already exercised your OTP, you can arrange with the current owners to allow you to engage a surveyor to do a topo survey of the house. this will come out with a proper plan of the existing house as well as the property's boundaries so you can see how much you can build your house to. for inter terrace houses, there is a 2m rear set back and a 7.5m front set back. unless URA has special planning permissions for your estate/area, then the set back might be different.
if the seller has the existing house's plans, get a copy from them. else try to see if they are willing to authorise you to purchase a set of building plans from BCA. ONLY building owners or their authorised personnel is allowed to purchase from BCA. even if you had exercised the OTP, you are not the legal owner yet until the completion of sale and purchase so you can't do it now.
go to PUB website and purchase a set of DIP and SIP for your house. this will let you know if your house is subjected to drainage reserves and/or whether there is a public sewer line running though your land. drainage reserves will affect your building's setback. if there is a sewer line running through your house, then you cannot build anything near or over the sewer line unless you build an RC trench to protect the sewer. depending on how deep and length of the sewer, this can add a few ten thousands or up to more than 100k for the RC trench.
Assuming that you can build another storey, then you may need to perform a soil investigation to check on the existing soil conditions. This will allow the professional engineer to determine whether the house's columns and beams need to be enlarged/strengthened to cater to the additional storey.
of cos you will need to find an architect and/or PE to work on your house's submissions. URA only accept submissions from registered architect or PE.
then you will also need to find a builder/contractor to build your house.
some builders offer all-in-one service but it's up to you to accept this or not. normally they will just get an architect or PE to do the submissions for a small fee. but of cos if anything happens, it's just you against the builder compared to when you engage an architect or PE directly, you have another party to project manage and protect your interest.
anyway, above should cover the basic steps needed before starting on the reconstruction. do note that there are quite a number of fees that need to be paid to authorities and such so do buffer at least 20k cash for all the fees.