Hello Abhishek, 

Sorry to say but with 18000 rank it's not possible to get admissions into NITs for B. Arch. 

IITs and NITs have their closing rank around 9k to 10k for the final rounds. 

As of now what you can do is to apply for your state's admission process. Also you can apply to some private institutes like Amity, LPU, Thappar etc. 

Hope this will help. 

Thanks.