flutter move floatingActionButton up 50 pixels

Wrap your FloatingActionButton inside a Padding and add the size you want:

    floatingActionButton: Padding(
            padding: const EdgeInsets.only(bottom: 50.0),
            child: FloatingActionButton(
              child: Icon(Icons.remove),
              onPressed: () => null,
            ),
          ), 

It's simple.

class Test extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Container(),
      floatingActionButton: Align(
          child: FloatingActionButton(onPressed: null),
          alignment: Alignment(1, 0.7)),
    );
  }
}

Use Alignment, as everything is a Widget in Flutter.