You can buy trailer hitch racks that’ll hold 7 bikes, if you don’t want to build+pull a trailer.

(this one’s 6, but I saw one in person just the other day with 7 on it)

A tiny car but 6 bikes. Sounds like an interesting story is lurking here.

(writing this in a slight rush; breakfast is in the oven; will follow-up later)

Answering the titular question, I don’t know where to obtain plans for such a multi-bike trailer. But having constructed a Harbor Freight trailer and pulling it using a small car to move my entire apartment, and have also used a motorcycle hitch carrier – as @The_Hideous_Orgalorg@sh.itjust.works suggested – to move an ebike, I have some considerations for you to take into account.

Starting with trailers, it looks like you’ve already looked into the vehicle registration requirements. I imagine they’re similar to what we have in California, in that the trailer must either have a manufacturer VIN or needs to obtain one through a process with the DMV. They would then inspect the trailer before assigning plates – if Washington State even needs plates for trailers; idk. Here in California, trailer plates are “permanent” but only in the sense that there are no annual registration stickers. Instead, they are renewed every five years, but based on the state’s fiscal years, rather than the month of initial registration. Many people even forget that they still have a trailer registered with DMV, and they only send a single notice two months before the 5-year renewal. Then again, it’s only like $25 for 5 years, so it’s hardly a massive burden.

For the physical trailer, those Harbor Freight 4x4 ft utility trailer are impressive for bulky-yet-light payloads, and bikes would meet that criteria. I have the 4x8 ft version, and used it to move all the furniture in my apartment in four round-trips. If you only load bikes, and construct it in such a way that it couldn’t realistically haul anything else, then there shouldn’t be a weight issue. All this assumes that the bikes would be properly secured to the trailer frame. The typical trailer requirements of properly positioning the payload are relaxed in this case, since bikes present little in weight – relative to the trailer itself; my 4x8 trailer is 300 lbs with 3/4 inch plywood surfacing – and not terribly much in wind resistance.

What you would still have to balance is the tongue weight of the trailer, upon the hitch of your small car. Larger trucks have higher tongue weight ratings, but smaller cars have less and need to be cautious not to let the trailer tail wag the car dog, to use a phrase. You cannot exceed the tongue weight rating of the car or its hitch – often 200 lbs max – but a zero or negative tongue weight would cause problems for your car’s rear suspension. A reasonable approximation for 0 < x < 200 lbs is to just lift the trailer off the hitch with one arm: if it’s takes merely a reasonable effort, then this is probably fine. If it’s too heavy to do with one hand, rearrange the trailer or the bikes. If it’s too light, do the same.

In addition to the registration effort, a trailer also imposes speed and handling limitations, although unlike a lifted pick-up truck, these limits are alleviated once you disconnect the trailer. Here in California, towing speed is less than full highway speed, being 55 MPH (88 kph) everywhere in the state, despite some highways being signed as high as 70 MPH (113 kph). The bearings on a Harbor Freight trailer might also not be fond of >55 MPH speeds, with some horror stories online of smoking bearings due to not keeping up with regular greasing.

Also, parking: vehicles with a shorter wheelbase will amplify steering input, which means manoeuvring into or out of a parking space will be tougher but not impossible. Supposing your car is 13 ft long, the hitch adds 1 ft, and the trailer is 8 ft, that’s 24 ft. In some more generous parking lots – like at home improvement stores – the parking bays are pained as 20 ft by 9 ft, but your trailer or car probably has 1-2 ft of overhang over the curb. So you’d still be sticking 2 ft out. If the places you go has trailer parking, then this is not an issue. But your turning circle will still be limited.

Going back instead to the hitch carrier idea, I’ve done this to move my ~100 lbs (45 kg) ebike, since I couldn’t find a bike carrier that was rated for that weight. However, while motorcycle carriers are readily available, they mostly only fit 2-inch square hitch receivers, and most small cars only have 1.25-inch square hitch receivers. An adapter would be necessary. Also, we go back to tongue weight: for a single acoustic bike, this is easy. For four road bikes, it might approach the same load as my ebike. But for six bikes, we need to rethink things.

Even if tongue weight were fine, the arrangement of the bikes on a hitch carrier have their own impact: moment of inertia. That is to say, if six bikes were arranged parallel to the drive axle, they’d have to each be spaced at least 8 inches (20 cm) apart. For six bikes, that means the carrier extends beyond the rear bumper by 48 inches (1.2 meters). And that’s just too much inertia to exert on a small car’s hitch receiver. Any bump that the car drives over would cause the bikes to also bounce, but that bounce then reflects itself due to the long moment arm, which then affects the car again. This is an identical wagging-the-dog scenario as trailers have, but here it’s a vertical wag rather than a lateral wag with a trailer. So this is way too long.

But perhaps you can go upwards, with vertically-arranged carriers. I’ve seen these on RVs, and on this page here: https://www.twowheelingtots.com/best-vertical-bike-racks-hitch/ . These have no extra registration fees, no additional road resistance from another axle, more manageable vertical inertia, and smaller impact for turning circles and parking. However, like with any hitch accessory, you just assure that your brakes lights (all three) remain visible, even in daylight. Obscuring brake lights becomes fodder for the insurance company to not fully pay a claim if someone rear-ends your car and bikes.

If you’re going to make something like that, perhaps skip the axle and construct it on something like this. No additional registration, less drag while hauling.

I’m in the state of Washington, do you guys know if I need annual registration and plates for something like this?