The reason most rental flats look bare is not a lack of money. It is that the expensive changes are all permanent and the cheap ones get skipped. A room can be made to look finished for very little if the spending goes to the three things that actually change how it reads, and nowhere else.

What the money is actually for

Three things do most of the work in any room. The rug, because it ties the furniture together and covers the worst of the floor. The curtains, because they change the height and the width of the window. The lighting, because it changes where the shadows fall.

Spend there first. Everything else is optional, and almost everything else is cheaper. A room with a correctly sized rug, curtains hung high and two lamps at different heights looks finished. A room with new cushions and no rug does not, however much was spent on it.

Where to spend almost nothing

• Moving furniture. Rearranging a room is free and changes it more than most purchases.
• Washing the curtains. Curtains that have never been washed look tired in a way that is hard to identify and easy to fix.
• Grouping what you own into odd numbers. Three objects in a cluster look deliberate, one looks like you only had one, two looks like a pair rather than a group.
• Lighting one lamp at a time to see which arrangement you actually prefer.
• Trimming a lampshade, or putting a lamp on a different base, which is free and reads as a completely different object.

The cheap things worth buying

Some categories genuinely hold up at the low end, and some genuinely do not.

Tension rods, over-door hooks, and removable adhesive pads are cheap, work, and come off the wall with nothing behind them. A rug from a discount retailer at the right size is the single best value in home decor. Cotton or linen curtains are hard to beat, because the expensive part of a curtain is often the heading rather than the fabric.

A ladder shelf is worth buying rather than improvising with an old bookcase, because a bookcase put against a wall is a bookcase, and a ladder shelf is visibly designed to lean.

Where to spend more than you planned

Two categories are worth paying properly for.

Mattress and pillows, because you use them every night and a bad one is felt immediately. And lamps, because a cheap lamp with a good bulb looks considerably better than an expensive one with a bad bulb, and the bulb is the part that decides.

Everything else is a place to be thrifty. Cushion covers in a neutral you already like can be changed seasonally or never. Picture frames are bought once and matter far less than how they are spaced. Vases and ornaments are the easiest place to save and the least noticeable.

The order to do it in

Do the free things first, because they change what you need to buy. Rearrange, then measure what is left, then decide what is actually missing. Most rooms need one or two things rather than a shopping trip, and the free work is usually what tells you which two.

The rug size guide and the curtain height guide are the two measurements worth getting right before spending anything, because getting them wrong means buying the wrong thing twice.