The first thing that needs to be done is to decide the nature of service you provide. It shall fall either under a Works Contract, a Composite Supply or a Mixed Supply.
Assuming it falls within the meaning of Works Contract, we have to see whether you have any "fixed establishment" in the state of Uttar Pradesh. If you do not have any such fixed establishment there you need not take a registration in that state.
However, if you do not take a registration in U.P. and if you purchase material from U.P. then you will not get the Input Credit of the GST paid on purchases.
Also, as per law there is a requirement to take registration in the above situation as a "Casual Taxable Person" in the state of U.P. Many experts say that the provision of taking registration as a CTP where ever we are providing the service is not the intention of law and hence we need not take this registration. Hence, not taking registration is not risk free.