Do you mean you personally are going to build the home? You will have to arrange financing, interim (construction) financing following by the financing after completion. Lots of details here to give attention to. You must have a good on-going arrangement with a financial partner. If you've done this before, I find it hard to believe you don't know what's involved. Many first time buyers think this should be easy--it's not.
Since you inquired about closing costs, also indicates perhaps you're dealing with a builder and 'custom' in this instance means one of the builder's plans adjusted to your needs. Most builders do offer options to many of their plans; giving the owner a home 'custom' to their needs. The builders alway have financing arrangements offered to prospects. The builder will tell you what they will pay or pickup as charges in arranging your financing and when going to their financing partner they are obligated to furnish as estimated closing statement. Ideally you should already have checked with lenders to see what your options are given your financial status. Please do not let everyone check your credit as your scores will fall. Run your own credit scores and have the 3 available when talking with banks/mortgage brokers.
Now please get represented by a licensed real estate agent/broker to assist in working with a builder. After Years of seeing homes built from 'scratch' to completion requires hands-on attention and follow up.