What you think as top-up-loan may be known as additional loan, second loan or enhancement of limit in bank's language. It depends on the purpose for which you want the loan amount.
Let us take the various situations.
1. If it is for additional construction, say additional floor or taking up an extra room which results in increase of the constructed area, then it may be an additional home loan(you may say top-up-loan). If you get a sanction from the authorities, and your till date payment record and your present disposable income also permits further repayment amount, then the bank may consider the loan under its terms and conditions and policy guidelines. It may be considered under home loan itself.
2. You are not going for any construction, but there needs some repair for the existing construction, or panting etc and the conditions of disposable extra income is satisfied, then the bank may consider it. Some banks may not consider it as housing loan, but a demand loan with separate terms of interest and repayment.
3. If the amount is needed for some miscellaneous purposes, then the loan may be considered under consumption loan or personal loan and will entail higher rate of interest and shorter repayment period.
For all this the bank may add the existing security to cover the new loan also. Some banks may consider further loan to the extent of amount already repaid. This is only in exceptional cases as per bank's discretion and experience with the borrower.
The first step is to have a personal discussion with the bank officials. Then you need to submit a formal application and add the latest income proof, latest statement of the existing loans and copy of the bank pas through which your income and transactions are routed. These are the minimum needed. Bank may ask or not ask further papers as per their assessment. You may have to remit the processing charges and other fees as applicable. If they want to take the house as security then they may need the latest tax paid receipt on it and the Nil encumbrance till date.
Then once sanctioned you may have to execute the loan documents which bank will provide you.