Our product is a web based course managemant system. We have 10+ clients and in future we may get more clients. (Asp.net,SQL Server)

Currently if one of our customers need extra functionality or customised business logic, we will change the db schema and code to meet the needs.

(we only have one branch code base and one database schema)

To make the change wont affect each others route, we use a client flag, which defined in a web config file, thus those extra fields and biz logic only applied to a particular customer's system.

if(ClientId = 'ABC')
{
   //DO ABC Stuff
}
else
{
   //Normal Route
}

One of our senior colleagues said, in this way, small company like us can save resources on supporting multiple resources.

But what I feel is, this strategy makes our code and database even harder to maintain.

Anyone there crossed similar situation? How do you handle that?