Not sure if the button is not closed or just you copied it here and forgot to close it.
#search-button {
width: 100px;
height: 50px;
}
#search-button svg {
width: 25px;
height: 25px;
}<button id="search-button">
<svg id="search-icon" class="search-icon" viewBox="0 0 24 24">
<path d="M15.5 14h-.79l-.28-.27C15.41 12.59 16 11.11 16 9.5 16 5.91 13.09 3 9.5 3S3 5.91 3 9.5 5.91 16 9.5 16c1.61 0 3.09-.59 4.23-1.57l.27.28v.79l5 4.99L20.49 19l-4.99-5zm-6 0C7.01 14 5 11.99 5 9.5S7.01 5 9.5 5 14 7.01 14 9.5 11.99 14 9.5 14z"/>
<path d="M0 0h24v24H0z" fill="none"/>
</svg>
</button>