How to read a floor plan

Most people read a floor plan by looking at the room labels and imagining the furniture. That tells you almost nothing about whether the house works. Here is the order a designer reads one in, and the questions worth asking at each step.

First, orient yourself

Before you look at a single room, find four things. They take thirty seconds and they change how everything else reads.

1. The scale. Usually in the title block: 1/4" = 1'-0" or 1:50. Without it you cannot judge whether a room is generous or cramped, and eyes are unreliable — a plan drawn small makes every room look cosy.
2. The north arrow. Which way the house faces decides which rooms get morning light, which get the afternoon heat, and whether that big south window is a blessing or an air-conditioning bill.
3. The overall dimensions. The outermost dimension line gives the footprint. Compare it against your plot before anything else.
4. Which floor you are looking at. Plans in a set look alike. Check the title before you wonder why the front door is missing.

Trace the outside wall

Follow the outer wall all the way round with your finger. You are after the shape of the building, not its rooms. A simple rectangle is cheap to build and easy to roof. Every step, corner and projection adds foundation, wall, roof junction and cost — and every one is a place water can get in. Complexity on a plan is complexity on site.

Find the front door, then walk the plan

This is the step that separates reading a plan from looking at one. Put yourself at the entrance and walk through it in your head, room by room, the way you would on an ordinary day.

• Arrive and put things down. Is there anywhere to stand out of the doorway, or does the front door open straight into the living room?
• Carry shopping to the kitchen. How far is it from the door, and does the route pass through a room where people are sitting?
• Go to the bathroom at night. From each bedroom — does the route cross a living space? Is there a toilet reachable by a guest without walking past bedroom doors?
• Get from the kitchen to the table. Carrying a hot dish, through a door you have to open.

Circulation problems never appear in a room label. They only appear when you walk the plan.

Now read the rooms

Room dimensions are given as width × length, and they are usually to the finished wall faces. Take the numbers off the drawing rather than eyeballing the boxes. Some useful yardsticks:

Then look at what the room has to hold. A 12' × 14' bedroom sounds fine until you place a king bed, two bedside tables and a wardrobe with doors that open. Sketch the furniture at the same scale; it is the only honest way to know.

Read the walls, not just the spaces

Wall length matters as much as floor area, because furniture and fittings go against walls. A room chopped up by three doors and a window may have nowhere to put a sofa, while a smaller room with two long uninterrupted walls takes furniture easily. Count the usable wall runs in each room.

The same applies to the openings themselves. Every door swing sterilises a quarter-circle of floor — see floor plan symbols for how swings are drawn. Windows dictate where a bed can go, since few people want to sleep under one.

Check the things that are easy to miss

• Does the stair line up between floors? An UP arrow on the ground floor needs a matching DN above it, in the same place. If the landing above lands in a bedroom, something has gone wrong.
• Do the plumbing rooms stack? Bathrooms above bathrooms, kitchen near the drainage run. Scattered wet rooms are expensive.
• Where do the windows actually look? A window three feet from the neighbour's wall is a light well, not a view, and it may be a privacy problem.
• Is there any storage? Cupboards are the first thing cut from a plan and the first thing missed in the house.
• Do the room dimensions add up? Room widths plus wall thicknesses along one face should equal the overall figure. If they do not, the drawing has an error in it.

Then look at the rest of the set

A plan alone cannot tell you about height. Ceiling heights, window sill and head heights, the pitch of the roof and how the stair climbs all live in the sections and elevations. If a plan is all you have been shown, you have seen about half the design. Section marks — the arrows on a line running across the plan — tell you where those cuts were taken.