How to set margin for a Button in Flutter

In Flutter 2.0, RaisedButton was deprecated, The ElevatedButton is replacement.
So you can use ElevatedButton and set style as bellow to remove margin:

      ElevatedButton.icon(
        onPressed: () {},
        icon: Icon(Icons.add),
        label: Text(
          'Add Place',
        ),
        style: ButtonStyle(tapTargetSize: MaterialTapTargetSize.shrinkWrap),
      ),

Alternatively you can wrap your button with Padding. (That is what Container does internally.)

Padding(
  padding: const EdgeInsets.all(20),
  child: ElevatedButton(
    onPressed: _submit,
    child: Text('Login'),
  ),
);

See this answer more more on adding margin to a widget.

Note: RaisedButton is deprecated as of Flutter 2.0. ElevatedButton is the replacement.

Put your button inside a Container and then set the margin

Container(
    margin: const EdgeInsets.only(top: 10.0),
    child : RaisedButton(
                onPressed: _submit,
                child: Text('Login'),
              ),