Here are a few ideas to get you inspired to use that extra space instead of wasting it:
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One of the most common reasons for it to not be used is because it's constantly shaded and depending on when the sun gets around to it, sometimes it's just plain dark and dingy. Adding a fireplace can make this unused space a go to for some warmth and comfort. This will also extend the life of your outdoor space!
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Screen it in! Keep the bugs out and enjoy some shade for a change. This will also open up the choices you can make decorating and furniture wise for the space!
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Make it a party space! Set it up for some great gatherings. This also doubles as a back up plan for outdoor parties in case of rainy weather.
Add some storage. Make the space have usable storage for all your outdoor needs. Whether it's under the stairs or along one wall, most outdoor spaces could use a little extra storage.
Light it up! Take advantage of the beams and the under side of your deck and make it the ultimate setting. Whether you simply add curtains from the beams and or string lights, this place can go from drab to romantic in minutes.