My house is approx. 60 years old it is a 1.5 storey house I would like to make it into a full 2 storey but will the 2x4 studs on the main floor be able to support the second floor or should I gut the entire house and put in 2x6 studs.
Shouldn't be a problem there are thousands of 2 story homes built using 2x4's.
One area of consideration is the headers over the existing windows and doors ... they may be under sized ....in cases they might be only be 2x6 headers instead of 2x8 , etc..
Hope this helps.:)
Another question to ask, is whether the foundation & soil will support the additional loads.
In most places adding a second story will require architectural or engineering stamps. Check with your local building department. They may have typical requirements to be used in your area.
Adding a second floor is generally not a DIY project.