Look at the line weights first. External walls are drawn as two heavy lines with a gap between them because they are hollow: a brick skin on the outside, a cavity, a timber or steel frame, then plasterboard. Internal walls are a thinner pair of lines. If an internal wall is drawn as thick as the outside ones, it is usually load bearing or a double brick party wall, and that is the wall you cannot move later without an engineer.

On a brick veneer house the outside wall is roughly a hand span thick once you add it all up. That matters when you are reading room sizes, because some plans quote frame to frame and some quote finished face to finished face. Ask which one you are looking at. The difference is enough to lose a bedside table.

Every door is drawn as a straight line with a quarter circle. The line is the door, the arc is where it swings. Trace the arcs with your finger and you will find the ones that clash: a bedroom door that hits the robe, an ensuite door that swings into the toilet, a laundry door that blocks the linen press. These get fixed for nothing on paper and for real money on site.

Doors are sized by their leaf width. A standard internal door in a volume build is 820mm, with 720mm often used on robes and small toilets. The number to watch is the bathroom. A 720 in an ensuite is tight for a walking frame or a wheelchair, and it is the door you will regret in twenty years. Many builders will swap to an 870 or 920 for very little, but only if you ask before the frame goes up.

The front door is usually 920 or 1020 wide and 2040 or 2340 high. A 2340 front door looks a lot better on a 2700 ceiling and a lot worse on a 2400 one, because it leaves almost no wall above it.

Minimum hallway width in a volume plan is often 1000mm between finished walls. It complies. It also means two people cannot pass without turning sideways, and a queen mattress goes around the corner on its edge with a scrape. A 1200 hallway feels normal. A 1350 feels generous and lets you hang art without it being at your shoulder.

Look at how much of the plan is hallway. A designer will tell you hallway is the most expensive floor in the house because it is square metres you build, heat and clean and never live in. Two metres of extra corridor is a walk in robe you did not get.

A bedroom marked 3.0 x 3.0 is a single bed and a desk. Put a queen bed in it and you will be climbing over the end to get to the other side. 3.0 x 3.3 takes a queen with bedside tables and a walkway one side. 3.6 x 3.6 is the size where a double bedroom stops feeling like a compromise. The main bedroom on most volume plans is 3.6 to 4.2 wide before you count the robe.

Check the robe depth. A robe needs 600mm clear inside to hang a coat hanger without it catching the door. Plans that draw a robe at 500 overall are giving you a shelf. Walk in robes need roughly 1600 wide for hanging one side and walking past, and 2100 or more for both sides.

The garage is the room people forget to measure. A single garage at 3.0 x 5.5 internal fits a hatchback and nothing else. A double at 5.5 x 5.5 fits two cars if you both fold your mirrors and nobody has a pram. 6.0 x 6.0 is where a double garage is actually a double garage. Look at the internal dimension, not the outside, and check where the pier or the hot water unit sits, because it is usually in the corner you wanted for the bins.

Windows carry a code like W04 or 1218. The four digit codes are height then width in centimetres: a 1218 is 1200 high by 1800 wide. Some builders use W numbers that point to a window schedule on another sheet. Either way, find the schedule, because a bedroom with an 0609 highlight window is a bedroom that looks at the ceiling.

Sill height is the bit nobody explains. A sill at 900 to 1000 above the floor puts your eye above it when you are sitting, and a bed can go under it. A sill at 600 gives a view from the couch but tells the street what is on your telly. Bathrooms and ensuites usually sit at 1500 or higher, or use obscure glass.

Now find the north arrow. It is usually small and near the title block. Living rooms and outdoor areas facing north get winter sun and can be shaded in summer. Big glass facing west cooks the room from three in the afternoon in Adelaide and Melbourne both. If the plan has the alfresco on the south and the garage on the north, the house has been drawn to fit the block, not the sun.

Ceiling height is not on the floor plan. It is on the sections and elevations, and sometimes only in the specification. 2400 is the old standard and it feels low with modern wide rooms. 2590 (often written 2.59 or "2600 nominal") is the current volume standard and reads as normal. 2700 is the upgrade that makes a house feel more expensive than it was, and it is much cheaper to buy at contract than to regret later.

A higher ceiling changes other numbers. Doors go from 2040 to 2340, robe carcasses get taller or leave a gap, and the bulkhead over the kitchen cabinets either grows or disappears. If you upgrade the ceiling, ask what else moves with it.