Design a parking lot
FreeLow-level designEasy
- Low level design
- Object oriented design
- Design patterns
Quick answer
This low-level design interview question asks you to design a parking lot. Aim for a design that is modelled as clear classes, interfaces and relationships that can handle new requirements without a rewrite. Interviewers want to see how you name responsibilities, keep classes small and reason about change.
The problem
Design the object model for a parking lot management system in a multi-level parking garage. A vehicle arrives at an entry gate, receives a ticket and is assigned a free spot that fits its size. When it leaves, the driver pays at an exit gate, the spot is freed and the ticket is closed.
What must a parking lot do?
- Levels and spots: The lot has several levels, and each level has many numbered spots.
- Spot sizes: Spots are small (motorcycles), compact (cars) or large (vans and trucks). Some compact spots also have an electric charger.
- Vehicle types: Motorcycle, car, van and truck. A vehicle may park in a spot of its own size or larger; a truck needs a large spot.
- Entry: At an entry gate, find a suitable free spot, hold it and issue a ticket with a ticket ID, the vehicle's plate number, the spot and the entry time. Refuse entry when no suitable spot is free.
- Exit and payment: At an exit gate, look up the ticket, compute the fee from the parked duration and the pricing rule, take payment by cash or card, free the spot and close the ticket.
- Availability display: Show the number of free spots of each size on every level and at the entrance.
- Lost ticket: Charge the daily maximum when a driver cannot show a ticket.
How well must it do it?
- Correct under concurrency: 10 entry gates and 10 exit gates work at the same time. Two gates must never assign the same spot.
- Fast assignment: Finding a spot must not scan every spot in the lot (up to 5 levels × 500 spots = 2,500 spots).
- Extensible: Adding a vehicle type, a spot type or a pricing rule (hourly, weekend flat rate, charging surcharge) should not require editing existing classes.
- Testable: Time and payment must be replaceable in tests so fee calculation can be unit tested.
Key concepts
- Low level design
- Object oriented design
- Design patterns
- Strategy pattern
- Concurrency
How you’ll practise
- Requirements
- Core objects
- Class relationships
- Key flows
- Extensibility
- Wrap-up
Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.
What do interviewers look for?
In any low-level design round.
- Lists the use cases before writing classes
- Gives each class one clear responsibility
- Uses interfaces where behaviour is likely to change
- Applies a design pattern only when it solves a real problem
- Handles edge cases and invalid states explicitly
- Walks through one use case end to end with the model
Questions people ask about this interview question
- Which design type is this question?
- It is a low-level design question. You design a parking lot and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Easy, so it suits engineers who are new to low-level design interviews. It is also a good warm-up before harder questions.
- What should I cover first?
- Start with the use cases and the core entities, then the responsibilities of each one. Agree on scope with the interviewer before you design anything.
- Can I practise it for free?
- Yes. It is one of the free questions. Sign up to practise it step by step. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.
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Updated · Written by the SevyDevy team