DIY Deck Extension ~ Deck Expansion Inspiration

DIY Deck Extension ~ Deck Expansion Inspiration

Is your deck or porch a little on the small side? Do you have room to add-on to it? We faced the same issue with our backyard deck and decided to do our own deck expansion. Keep reading to find out the steps we took for our DIY deck extension.

Outdoor furniture for deck extension

Whether for a party, a BBQ, or just to relax…nothing is better about the backyard than a roomy outdoor area. Since we bought this house a year ago, I’ve been dissatisfied with the back deck situation. It wasn’t a tiny deck, but I just didn’t feel I could entertain well on it. There definitely wasn’t room for the grill, smoker, and mini-fridge along with the patio furniture.

Initially I thought about adding another lower-level deck off the existing one, but decided I wouldn’t be completely satisfied with that and would rather do a deck extension instead. We checked around and the cost to hire someone, along with the time frame was outrageous so we decided to tackle it ourselves!

We aren’t experts in this matter and haven’t tackled a project this large before. We did some initial research on methods and products, made a list of supplies, and headed to Lowe’s with the intention of getting everything we needed in one trip. Ha! Yeah right, not so much….I think over the course of this project, we made 4-5 trips to get more things!

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How To Complete a Deck Extension

Supplies & Tools

(These are what we used for our project and may vary for your project)

  • Concrete deck blocks
  • 4×4’s
  • 2×8’s
  • Deck boards
  • Exterior Screws
  • Joist Hangers
  • Masonry String


1)  Remove Balusters, Railing, Posts

In order to add on, we needed to get rid of the balusters, railing and posts that were part of the original deck. The balusters were a joke and we were able to just tug slightly on them and they came loose. We used the drill to unscrew the handrails and then used a reciprocating saw to cut off the posts flush with the original deck framing.

Old Deck during demo for deck extension

2) Squaring, Setting, & Leveling the Deck Blocks

We measured 6ft. out from the existing deck and marked it off. The width of our expansion was 16ft. so we lined up a concrete deck block 6 ft. out from each corner and a third at the center point.

(Another option is to dig holes and cement the deck posts into the ground. We likely would have gone this route if we were building a deck from scratch, but since we were just extending it out 6ft. and the ground in our yard is very hard/compact, we opted to use the deck blocks.)

Concrete deck blocks for deck extension

We set a 4X4 in each deck block and used string to check that we were squared up with the existing structure. This was accomplished by attaching the string to the existing decks fascia board and ensuring the string stayed flush against the board while we aligned the new 4×4 posts.

Once we ensured the placement of the deck blocks, we used a shovel to remove the grass and top layer of dirt and leveled out the blocks in place.

3) Joist & Posts

For the deck joists, we bought 2x8x12’s and essentially cut them half with the miter saw. We used a joist hanger & joist nails to attach the first joist on the outer edge to the existing deck and leveled it by putting a temporary scrap wood brace at the far end. Once we’d done that, we measured how tall our first corner 4×4 post would need to be and cut it to size.

Metal Joist Hanger for Deck extension

We notched the top portion of the 4×4 post with the circular saw & jigsaw so the 2×8 joist could rest on it. It was also notched for the long 2x8x16 face board that runs the width of the extension. Once the first post was done, we screwed the end of that first joist to it.

Notching the 4×4’s for the deck framing to rest on, strengthens the foundation of the deck. We did this same process for the joist on the other side of the deck too.

Notched Corner Deck Post for deck extension

With the other side of the deck, we were extending the width of the deck and we needed to attach the far joist to the brick of the house. So we used masonry anchors to attach a section of 2×8 flat against the brick, then we used a joist hanger to attach the joist to it.

Next, we attached the 2x8x16 face board, resting it on the notched posts and screwing it into place. We  used this to measure the height of our center post and we only notched the post along the top outside for the face board to rest on.

At this point, the outside frame of the extension was done, so we were able to space out the remaining joists and attach them at both ends using the joist hangers and joist nails. We checked each joist was level before securing it.

4) Extra Support

Since we didn’t build the original deck and couldn’t vouch for it’s structural support, we didn’t feel comfortable adding the weight of the extension without additional support. We decided to run another 2x8x16 board under the new joists and close to the original deck structure.

Additional Cross Beam Deck Support for deck extension

We came out about a foot from the original deck posts and leveled/set 3 more deck blocks in place. We measured out 3 more 4×4 posts and notched each for the 2x8x16 to rest on. We attached it to the posts with screws and then attached the 2x8x16 to the crossing joists using twist straps.

We also used smaller pieces of 2×8 to add extra bracing and support between the joists surrounding the deck support posts.

Extra Bracing for the deck posts for deck extension

5) Deck Boards

After that we were able to start placing the deck boards and screwing them into position. Since the boards are never perfect and some are a little bowed, we used a longer 2×4 to wedge against them for leverage to position them as tightly together as possible. The boards will shrink some and will cause some gaps, so it was best to place them without a gap.

Deck Floor for deck extension

Once all the boards were screwed down, we used the circular saw to trim off the edges of the deck boards to make an even edge, so we could go back and screw a fascia board along the two sides to cover the appearance of all the ends of the deck boards.

We also replaced several of the existing deck boards that were in rough shape.

6) Handrail

4x4 Deck posts for deck extension Tall 4x4 posts for deck extension

We did the handrail a bit differently. We wanted to be able to put up some sort of shade covering above the new deck portion and so we needed tall posts. Instead of having continuous posts from the deck blocks up to the height of the handrail, we chose to add taller 4×4’s and bolt them to the support posts at the outside edge of the deck. The posts we added on the far side of the deck were standard height and were also bolted in place.

Deck Handrail for deck extension

To construct the handrail, we screwed 2×4’s together in an ‘L’ shape and attached them between the posts using pocket holes made with the Kreg Jig. You can learn more on how to use a Kreg Jig here. We then screwed a deck board on top of those supports as the top of the handrail. We notched the deck board to fit around the 4×4 posts. We put the handrail at about 36” high.

Handrail notched for post for deck extension

Instead of adding back a ton of 2×2 balusters, we wanted to do cable railing. We used a scrap board and made a jig with pre-drilled holes spaced evenly apart and used it to drill holes through the railing posts.

We put eye hooks into the posts far opposite each other and on one end we added a turnbuckle. We used a ferrule and a swaging tool to crimp them closed and keep the cable from slipping.

Swaging Tool and Ferrule kit for cable rail for deck extension

Turnbuckle for deck extension

We then ran the cable through the pre-drilled holes in the posts and around to the other eye hook and used another crimped ferrule to secure the cable on that end. The turnbuckle was used to further tighten the cable. We repeated this for 2 more rows of cable.

Finalized deck extension construction

And ta-da! A larger deck space! It was definitely a long, hot, sweaty project but we are very proud of our accomplishments and the money we were able to save by spending a few weekends on our own DIY deck extension.

Since we hadn’t done this before, there were definitely some struggles and re-do’s along the way, but we managed and couldn’t be happier! We increased our deck space by roughly 100 sqft.  Please share if you have tackled your own deck extension…I’d love to see pics!

Until next time,

Into DIY signature

P.S. – This is part 1 of a 5 part series for our overall deck project.

Part 2 – DIY deck shade

Part 3 – DIY hinged deck rail dining

Part 4 – How to remove a  horrid stain job off the existing deck and clean/prep the entire deck for stain

Part 5 –  How to stain a deck

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Deck Expansion DIY
Need a bigger deck? See how we did a DIY deck extension and added 100 sqft of space to our deck!

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