You are right, Theatre road is indeed a hotspot area and it is quite challenging to get an empty plot to build a guest house as you have suggested. Since paying house is a commercial activity, the price you will be paying to built and finally operate the guest house might not equate to the gains you may get. There are a number of accommodation options in the region and competition is another aspect which you need to focus before finalizing any land deals. Getting an empty plot in Theatre road is a tough ask but you may get some deals from resellers although at premium prices.
I agree with Mr Sujan and Javed that Theatre road is one of the hot spot in Kolkata and finding a property in this locality is a tough job. By the way what is your total budget?
Theatre Road also known as Shakespeare Sarani is one of the most famous locations in central Kolkata. It may be difficult for you to buy an adequate piece of land at a decent price. The street is always bustling with activity and is considered a prime locality of the city. Close proximity to various commercial centers and the port makes Theatre Road one of the best areas to have a project that you are planning but either the prices may be just too steep or you may not get adequate land as per your choice. Property rates in Theatre road range between Rs 10,000 to Rs 12, 000 per square feet.