diff --git a/rfcs/0001-type-extensibility.md b/rfcs/0001-type-extensibility.md
new file mode 100644
index 0000000000..0426cf9efc
--- /dev/null
+++ b/rfcs/0001-type-extensibility.md
@@ -0,0 +1,97 @@
+- 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 JavaScript 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. Mainly the `build` function.
+
+A custom types `Builder`-module shall extend another types builder or the generic `AbstractBuilder` and overwrite relevant functions:
+```js
+class MyCustomBuilder extends AbstractBuilder {
+    build() {
+        super.build();
+        myCustomBuildTask();
+    }
+}
+```
+
+### Generic handling of extension
+A "Type Extension" will be only one way to extend the UI5 Build and Development Tooling. Other possible extensions include "Shims" (see RFC 0002), server middlewares, translators.
+
+Therefore a somewhat generic concept for dealing with extensions is needed.
+
+To separate "UI5 Projects" (i.e. things that represent UI5-artifacts for the browser) from tooling specific things like "extensions", an additional attribute "kind" is added to the ui5.yaml.
+
+
+#### Example type extension
+```yaml
+specVersion: "0.1"
+kind: extension
+type: project-type
+metadata:
+    name: my-custom-library
+```
+
+#### Example library
+```yaml
+specVersion: "0.1"
+kind: project
+type: my-custom-library
+metadata:
+    name: my.application
+```
+
+The `kind` attribute defaults to `project`.
+
+### Collecting and applying type extensions
+A type extension might be a standalone module or part of a project. In the above "type extension"/"library" example, the library declares a dependency to the type extension module. `ui5-project` should resolve the dependency, identify it as an extension and run the appropriate formatter/type for the extension type. This will then add the type as "my-custom-library" to the type repository.
+
+If the type extension is part of a project, the single `ui5.yaml` for above example looks like this:
+
+```yaml
+specVersion: "0.1"
+kind: project
+type: my-custom-library
+metadata:
+    name: my.application
+----
+specVersion: "0.1"
+kind: extension
+type: project-type
+metadata:
+    name: my-custom-library
+```
+
+In this case the type extension is no dependency of any kind but automatically collected and processed with the processing of the project.
+
+## 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