Your pre-approaval only applies to the loan type you requested. If you change loan types you must be pre-approved again fot that specific loan type.
The most favorable option for you is called a "Construction/Premanent" or "One Step" loan.
This loan combines both the construction loan and the permanent loan in one package so that you are not paying two sets of loan fees. Not all lenders offer this option so you may have to shop for it. If you will let me know which state you are in I can help you find a lender who offers this program.
Your minimum down payment will be 5% of the sales price of the land and proposed construction costs combined. In addition, the lender will require you have sufficient funds for closing costs and, some lenders may require you have a reserve fund sufficient to cover any overruns or upgrades. This reserve fun does not have to be liquid. It could be your 401K. Some lenders will allow you to finance the estimated 5% reserve requirement if you put down a 10% downpayment.
The loan works like this:
The appraisal is completed using plans and specs to determine completed value.
Your builder supplies materials specifications and a construction/draw schedule.
The draws against the loan are given to the builder based either on percentage of completion or on a line item basis. Most lenders make the draw checks jointly payable to you and the builder so that you may maintain some control.
You pay interest only payments on the funds as they are disbursed to the builder at whatever rate the construction term is set for.
At the end of construction the loan converts to a permanent loan. This process will vary a bit from lender to lender. Some convert you at the same rate as your construction loan, some offfer you a float down option, lots of vartiety here.
The normal construction term will range from 6 months to a year or longer on a big project.