Service Tax Registration for New Service

One of my client is providing Management Consultancy services for the last 4-5 years in the name of his sole proprietory firm and is already having ST Registration in the name of his Sole Proprietory Firm. The value of taxable services during fin. year 2008-09 and 2009-10 were less than Rs.6.00 lacs each year, hence no service tax liability.
- for a registered assessee, its mandatory to file the NIL return within prescribed time limit, failing which upfront penalty is leviable not exceeding Rs 2000 per return period. 

Now he has purchased one commercial office and given it on monthly rent of Rs. 20,000/- w.e.f. 1st December 2010. Is he required to get this rental service also registered under service tax or not? If yes, will it be in his individual name or he has to get endorsement on his existing ST Registeration in the name of Sole Proprietory Firm. During current financial year, total value of services provided/ to be provided including Rental income will be less than Rs.9.00 lacs.
- He can get the same endorsed on existing registration certificate, as proprietorship firms has pan of proprietor only, so individual or firm , same identity.

Is there any time limit for applying for new ST registration or for endorsement in existing ST Registration? Please clarify.
- as the assesse is already registered, so he can apply for endorsement immediately on commence of new service, not exceeding the current return period, 

- if assessee is liable to pay service tax than 30 days. 

Thanks