Single storey house can be renovated to 1 1/2 storey.  Even if u don have the budget now, it is worth to incorporate that in your plan, and do that as a second phase project.  Even if u sell the house, the structural work u put in shld add value.

If it is an old house, it is advisable to redo the wiring and the plumbing.  My timber floor supplier told me his fren house renovated without redoing the old pipes, and only did the new areas.  Some time later his water bill shot thru the roof, then only discover the pipe burst!!  Will be costly u know to rectify it.

When we renovate we like to knock down walls etc.  Must find out where the old beams and load bearing columns are and work.  I used a mppj draftsman to do my plan, in the end disaster.  The planned bedroom ended up with 2 load bearing column, 1 at the bed location and the other at the wardrobe location.  In the end, I had to hastily changed the layout... 

Decide before hand your lighting plan.  Research on what u want, and whether u will have plaster ceiling.  When engaging electrician, don just tell them where u want your lights, tell them whether u want your switches too, otherwise, u can be rudely surprised.  Walk around each room and visualise where u wld be needing light points and sockets.

Renovation is expensive, but I suppose u can cut down cost by sourcing your own materials.  U may spend the same amt of money, but you are probably getting better specs and quality.

Also be realistic about what you can afford and spend within that.  It is important to know ahead what u going to need so that the project wont be abandoned!!!  If possible, arrange for standby credit line, by way of refinancing your home for a higher value (asssuming value has appreciated).

Don't forget to apply approval plan and renovation permit from
Majlis Perbandaran..that's also include on ur costing

there's alot of info in LYN... make sure u checked them all out before u proceed.... good luck