depending on what you call it a web app, the answer could be different. But if what you meant is in the most common sense, then the wep app would refer to the situation that user uses a web browser to sent request (entering Url in address bar or click links on the web page), and then a web server in the world somewhere sent a stream of data back to the BROWER, then the browser translate the data stream into viewable web page for the user.
As you can see, from user point of view, everything runs inside a browser, in which AutoCAD cannot run. So, no, in this sense, you cannot develop a web app.
On the other hand, AutoCAD can run on a computer, even it is mainly used as web server (well, if security is not of concern). So, technically, you can automate AutoCAD on server side, and have a web server receive user request as input and pass it to automated Acad session, do something, such as process a drawing, and then returned the process result back to web user, such as letting user to download processed drawing. However, it is very likely the Acad lisence does not aloow this type of use of Acad. Also, Acad is designed as desktop app and heavily it heavily relies on user interaction and is a resource hog. Running it on server side with user watching it is hardly a dependable operation.
There is other way to do Acad drawing replated web app, such as using RealDwg to process drawing on the server side without using AutoCAD. But if your goal is to develop Acad add-in with .NET API and secure your code from being decompiled, the looking at web app would be wrong direction, IMO.
