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DIY Alien Space Detective Costume

Halloween Costumes | Sewing

Learn how to make an alien space detective costume for Halloween! With some fun galaxy fabric, a Butterick sewing pattern and some fabric markers I was able to create a fun Halloween costume!

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Detective coat made from blue galaxy fabric with a green and blue plaid cape and deerstalker cap.

Hello Crafty Friends! I’m back with another fun Halloween costume, this time an alien space detective! When my son first asked to go as an alien space detective I had a lot of questions – like what is that?? Is it an alien who is a detective or a detective investigating aliens? Either way I’m always up for a costume challenge and had a lot of fun creating this costume! 

In this post I’m sharing how this costume came together along with the supplies and patterns I used. I hope it may help if you find yourself tasked with making something similar!

Deerstalker cap colored with green and blue markers.

Alien Space Detective Costume

Materials and Supplies

The Fabrics

For this alien space detective I decided to look to famous detectives like Sherlock Holmes for inspiration. I wanted the costume to be somewhat recognizable so figured this was a good place to start. When I think of Holmes, I usually picture an inverness coat and deerstalker cap. So I imagined if you were a detective of aliens on another planet, how could these iconic pieces of clothing be altered – the answer I came up with was to switch-up the fabrics.

I ditched Holmes traditional brown tweeds for vibrant, colorful fabrics in blue and neon green. Since we have been known to have snow at Halloween, I decided to use fleece as the base of the costume. I picked out a blue/green cotton fabric in a sparkly cosmic print to use for the coat and found the perfect plaid flannel in glue and neon green. 

Fabrics used to make the coat including a blue fleece, blue and green plaid flannel and sparkly glue galaxy printed cotton.

The Coat

Sherlock Holmes is often pictured wearing an inverness coat or inverness cape – basically a long coat with a short cape attached to the top. Usually the coat does not have sleeves but the cape covers the top part of the wearer’s arms. They were popular during the late 19th century and usually worn as overcoats.

I created a coat that resembles an inverness coat with the signature cape attached but made my cape short and included sleeves on my coat. Since this is an alien detective I could really do anything. There are no rules with Halloween costumes! 

The coat was created using the Butterick pattern 3244. I used the same firefighter coat pattern that I used a couple years ago when my son went as a firefighter for Halloween. It was much easier and faster this time around because I didn’t add the black bands or latch closures.

Butterick Pattern 3244 lays on a stack of blue and green fabrics.

Since the cotton was too thin on it’s own, I flat-lined it to the fleece fabric. This meant I cut out the pattern first from the cotton then pinned the cotton pieces to fleece and cut out the fleece. I stitched the pieces together close to the edges to make the two pieces of fabric act as one while sewing the coat. 

To keep things simple the coat closes with a long strip of Velcro down the front to hold it closed. Afterwards, I added buttons to the front of the coat for decoration.

The detective coat is opened showing a black velcro closure.

Adding the cape part of the coat was a bit of a guessing game. I made something that resembled a small circle skirt from the plaid fabric, then tried it on my son and kept making adjustments until it looked okay. Once it was figured out and sewn, I stitched it to the neckline of the coat before adding the collar. It was a bit trickier than I anticipated and took time to get it to look okay. 

Detective coat made from blue galaxy fabric with a green and blue plaid cape.

The Deerstalker Cap

A detective costume wouldn’t be complete without a classic deerstalker cap! This type of hat has duck bills in the front and back of the cap. This would protect the wearer while they were out hunting or fishing.

The cap can also have two flaps that come down to cover the ears with grosgrain (a ribbed ribbon) attached to the bottom ends. These could be tied under the chin to protect the wearer from the cold. When not in use, they are usually tied up with a grosgrain bow on top of the cap.

Deerstalker cap colored with green and blue markers.

To make an outer space version of this cap, I purchased a deerstalker cap on Amazon and decided to color it to match the coat. I actually ordered the hat in a couple of sizes and picked the one that fit best. The “child” size was too small so I was happy I ordered the adult size small which fit great.

I was originally going to order two baseball caps and put them together to create a detective hat like Martha Stewart does in this tutorial – but two baseball hats were the same price as the actual deerstalker hat on sale, so ordering the deerstalker hat seemed like the best option!


Collage showing a deerstalker cap and the same one colored with blue and green markers.

To give the hat its “outer space” makeover, I used blue and green fabric markers to change its look. It was easy to color in every other white diagonal line, alternating between green and blue. This was a great project to do while watching a movie!

Fabric markers sit next to a deerstalker cap and the white parts are colored in with blue and green markers.

Next, I covered the ear flaps with the same green and blue plaid fabric I used to make the cape. Because I just covered the outside I let my son know he’d need to keep the flaps tied up.

Green and blue fabric is wrapped around the deerstalker flaps.

It turned out so well and was definitely easier than attempting to create a similar hat from scratch! Plus, years later my kids still use this hat for pretend play.

Deerstalker cap colored with green and blue markers.

The final touch was adding a blue magnifying glass we had from our science supplies, and the costume was finished! This was another super-fun Halloween costume I was able to make for my kiddos. I love trying to create their crazy ideas!

Detective coat made from blue galaxy fabric with a green and blue plaid cape and deerstalker cap.

I hope you enjoyed checking out this fun alien space detective costume! Check out some of my other handmade costume on my DIY Costume Page. Happy Halloween!

Happy Crafting! -Kim 

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