What I want to do is to expand the bounds of a Google Maps by a certain distance. This is because I want to include markers that are outside the bounds of the map since they have overlays attached to them which will overlap onto the current map bounds.
For example, if I am currently viewing the green area on the map below, I want to include all markers in the red area.
Therefore, after getting the bounds from Google Maps I need to extend the bounds by a certain distance to get a new set of bounds.

Here is how I have done it.

For longitude:

For Latitude:

The Javascript

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    // returns latlng of new coordinate which is dist from the longitude given

    function lngDistance(lat1, lng1, dist) {

        var R = 6371000; // m

        lat1 = deg2rad(lat1);

        lng1 = deg2rad(lng1);

        var lng2 = (R*lng1*Math.cos(lat1)-dist)/(R*Math.cos(lat1));

        return(rad2lng(lng2));

    }



    // returns latlng of new coordinate which is dist from the latitude given

    function latDistance(lat1, dist) {

        var R = 6371000; // m

        var lat1 = deg2rad(lat1);

        var lat2 = (R*lat1-dist)/R;

        return(rad2lat(lat2));

    }



    function extendBounds(bounds,dist) {

        var latNE = bounds.getNorthEast().lat();

        var lngNE = bounds.getNorthEast().lng();

        

        var latSW = bounds.getSouthWest().lat();

        var lngSW = bounds.getSouthWest().lng();

        var latlngNE = new google.maps.LatLng(latDistance(latNE, -dist), lngDistance(latNE,lngNE, -dist));

        var latlngSW = new google.maps.LatLng(latDistance(latSW, dist), lngDistance(latSW, lngSW, dist));

        return (new google.maps.LatLngBounds(latlngSW, latlngNE));

    }

Not that I correct latlngs using the rad2lat and rad2lng functions given in my post converting radians in degrees latitude and longitude