Capital gains used for house construction

You are correct, you may deposit the sell amount in the CGAS account and as and when required can spent it from that account. By this way, you will be able to save the LTCG u/s. 54F from the land sold.

The amount should be used up or deposited to CGAS account before filing relevant ITR

Yes, you should consider putting the amount you received from selling your land into a capital gain account. This account can help you defer taxes on the capital gains. You can then use the funds from that account to construct your new house.