from typing import List


def extend(layout: List[List[str]]) -> List[List[str]]:
    """
    Extend layout by adding a border of empty seats

    :param layout: initial layout
    :return: layout with a border of empty seats
    """

    empty_row = ['.'] * (2 + len(layout[0]))
    retval = list()
    retval.append(empty_row)
    for r in layout:
        r.insert(0, '.')
        r.insert(len(r), '.')
        retval.append(r)
    retval.append(empty_row)

    return retval