diff --git a/rfcs/0001-type-extensibility.md b/rfcs/0001-type-extensibility.md
new file mode 100644
index 000000000..5d549b15c
--- /dev/null
+++ b/rfcs/0001-type-extensibility.md
@@ -0,0 +1,74 @@
+- Start Date: 2018-04-03
+- RFC PR: [#4](https://github.com/SAP/ui5-tooling/pull/4)
+- Issue: -
+- Affected components
+    + [x] [ui5-builder](https://github.com/SAP/ui5-builder)
+    + [ ] [ui5-server](https://github.com/SAP/ui5-server)
+    + [ ] [ui5-cli](https://github.com/SAP/ui5-cli)
+    + [ ] [ui5-fs](https://github.com/SAP/ui5-fs)
+    + [x] [ui5-project](https://github.com/SAP/ui5-project)
+    + [ ] [ui5-logger](https://github.com/SAP/ui5-logger)
+
+# RFC 0001 Type Extensibility
+## Summary
+Add a feature to customize how a specific UI5 project is being built.
+
+## Motivation
+Currently the UI5 build is only capable of building UI5 projects of types "application" and "library" with a fixed set of tasks it executes.
+
+A UI5 project (for example a library) may want to add or modify build steps (for example execute custom bundling). For this, an extensibility mechanism is needed.
+
+Multiple UI5 projects may require the same kind of "customized" build. Therefore reuse should be possible.
+
+## Detailed design
+### Extensibility of types
+Leverage the existing mechanism of types (currently for application and library), allow for a custom type and do the necessary adaption via regular JS language features (i.e. object-orientation, deriving and overriding).
+
+The `AbstractBuilder` (as well as any subclass-implementation) offers a set of functions which can be overwritten.
+
+#### Combined list of tasks executed by application- and library types
+- replaceCopyright
+- replaceVersion
+- buildThemes
+- generateLibraryPreload
+- createDebugFiles
+- uglify
+- generateFlexChangesBundle
+- generateManifestBundle
+- generateAppPreload
+- generateStandaloneAppBundle
+- generateVersionInfo
+
+#### Functions of the `AbstractBuilder` ("Phases")
+1. preprocess()
+    - replaceCopyright
+    - replaceVersion
+1. process()
+    - createDebugFiles()
+    - buildThemes()
+1. bundle()
+1. postprocess() / optimize()
+    - uglify()
+
+A custom types `Builder` module shall extend another types builder or the generic `AbstractBuilder`:
+```js
+class MyCustomBuilder extends AbstractBuilder {
+    bundle() {
+        super.bundle();
+        myCustomBundle();
+    }
+}
+```
+
+### Collecting and applying type extensions
+
+## How we teach this
+TBD
+
+## Drawbacks
+TBD
+
+## Alternatives
+There are ways to consume (and thereby possibly adapt) the existing tooling through its API via taskrunners such as grunt or gulp, or using a custom node.js script. But this offers only limited possibilities, especially when it comes to building transient dependencies.
+
+## Unresolved questions