However, if you are using a new programming language or don't have a good history of past estimates, then the best method would be to keep decomposing the work into smaller, easily estimated pieces, estimate those, and then roll the total back up.

If you have a previously made component that is similar in size and scope, then you can count that. If you have good historical projects, then you can use estimation by analogy or proxy-based estimates to compare with previous projects or components. If you do, you can use any number of techniques (including the other ones that I mention) as an individual contribution to the group estimating effort. For example, if you don't have a group of knowledgable people, you really can't use expert judgement in groups. The method you choose is likely to be determined by your environment. You can consider decomposition and recomposition, estimation by analogy, proxy-based estimates, and expert judgement in groups to estimate the size to use as an input into COCOMO. There are different estimation approaches that you can use to try to come up with an estimate for the size of the software application under design. The only way to get the SLOC input to COCOMO is to estimate that.

Categories of cocomo model