An Engineer's Side-Business Rental Spaces: ¥7.5 Million in Year One Across Three Locations. Measured Unit Economics of a 25m², ¥450,000-Upfront Space

The published numbers (one multi-purpose space in Saitama)

How a full-time engineer chose a side business that doesn’t sell his time

Hirosena’s day job is engineering in web development and B2B SaaS; he heads the IT company More Inc. and formerly worked at Recruit’s tech arm. He has two children, and in the note published in October 2024 he describes wanting to prioritize family time as the motive behind his choice of side business. Contract development, the default side gig for engineers, is selling time by the slice, which conflicts with that goal. In 2023 he chose rental-space operation instead, and in the following year’s note announced, “I achieved about ¥7.5 million in annual side income in my first year.” He positions the business as sitting “between labor-type and investment-type” income — meaning that once the initial investment and setup labor are done, it keeps running on a thin layer of ongoing work.

The core of the business: arbitrage on subletting

The structure of a rental-space side business is an arbitrage: rent an apartment on a lease, then sublet it by the hour (using a property where subletting is permitted). At the published Saitama location, a 25m² room rented for ¥91,600 a month is sold at around ¥3,000 per hour. Monthly revenue of ¥250,000 equals about 80 booked hours, just under 3 hours of utilization a day generating 2.7 times the rent.

Platforms like Space Market handle nearly all customer acquisition, and smart locks make the space unmanned. The operator’s job compresses down to “an eye for location × use case” and “photos and copy that win on the listing screen.” Just as Tokuda on minne invested in photography, competition inside a platform is decided on the results page.

These measured figures can also be checked against industry benchmarks. A market-rate document for the same industry puts standard utilization at 60–80 hours a month and the core hourly band at ¥3,000–6,000. The Saitama location’s ¥250,000 monthly revenue (about 80 hours × roughly ¥3,000) matches exactly the product of the top of the standard utilization band and the bottom of the rate band. Not an inflated number, but the level of a standard space run properly to its full. That reading is what makes this published data so usable.

The shortcut at launch — choosing a business transfer

Easily overlooked: the Saitama location was not built from scratch. He started operations by taking over a property that was already a rental space via business transfer, beginning with the interior, furniture, and listing track record already in place. The deepest valley in an hourly-rental business is the period right after opening (no bookings means no reviews, and no reviews means no bookings) and starting via a transfer shortcuts that valley entirely. The lightness of the numbers (¥450,000 upfront, payback in about six months) is very likely helped by this starting format. Anyone launching from zero should budget heavier figures than these.

Inside the 10 minutes a day — what he kept and what he let go

The only thing Hirosena handles himself is customer support. He writes that “the side business takes about 10 minutes a day.” Booking management is wired up across multiple platforms to prevent double bookings, and revenue tracking and cleaning are outsourced. The ¥7.5 million is not the product of labor volume but the result of designing the systems first and leaving none of his own labor in the loop.

His differentiation moves are concrete, too. At the Saitama location, after analyzing nearby competing spaces, he installed more than 20 board games and runs a student-only discount. Rather than relying on one-off party bookings, capturing regular users like English-conversation classes or yoga classes stabilizes the utilization baseline, a repeat-customer playbook he cites himself. Hourly rental is an inventory business where “an unsold hour vanishes,” and regular users directly reduce vacancy risk.

How to read the ¥7.5 million

“About ¥7.5 million in first-year side income” is an eye-catching number, but it needs careful reading. The original wording is “side income” (fukushūnyū), with no explicit statement of whether it means revenue or profit. The published Saitama location’s monthly profit is ¥68,500, if all three locations were at that level, annual profit would come to just under ¥2.5 million. On a revenue basis, however, ¥250,000 × 3 locations × 12 months = on the order of ¥9 million, which is consistent with ¥7.5 million. The other two locations are in central Tokyo and their numbers are not public, so nothing can be stated definitively, but it is safer to read the ¥7.5 million as a figure closer to revenue than to profit.

The measured figure that carries more weight is actually the “modesty” of ¥68,500 in monthly profit per location. One location alone is underwhelming as side income. Which is precisely why, if the template of ¥450,000 upfront and six-month payback is repeatable, the number of locations becomes the income multiplier. The ¥7.5 million is not one location’s jackpot but the result of copying the template.

Risks, and what to discount

The biggest risk is not vacancy but the “subletting premise” itself. The three main variables are securing a sublease-permitted property, neighborhood trouble (noise), and platform fee or policy changes, and for party use in particular, noise complaints lead directly to eviction. The structure whereby the profit margin (¥250,000 revenue → ¥68,500 profit, about 27%) is squeezed by rent and fees also shows how little pricing freedom the operator has. He himself lists the difficulty of securing properties, customer acquisition, revenue plateaus, and customer trouble as challenges of the business.

One more thing: this note includes a funnel into a rental-space operators’ community that he runs. That he publishes from a position with an incentive to make entry look attractive is worth deducting when you read the numbers.

Conditions for replication

“¥450,000 upfront, payback in six months” is unusually light for an offline side business. Compared with a food truck’s several-million-yen vehicle, the cost of retreat on failure (restoration plus penalties) is an order of magnitude smaller, so the small-start principle (test small, learn the template, then copy it) applies directly. But the preconditions for replication are narrow: being able to reach a sublease-permitted property, shortcutting the launch valley via business transfer or a furnished takeover, and systematizing bookings and cleaning so that none of your own labor remains. Entries missing these three tend to fall on the side of the “roughly 30% in the red in year one” shown by the same market’s distribution data (the four-location, ¥360,000-monthly-profit case).

Related reading

• Four months of a food truck — the heavy end of the same offline side-business spectrum
• The four-location, ¥360,000-monthly-profit case — market-rate data for the same industry