If most or all of the rooms in your home look small and cramped, here are two things you can do to make them look and feel more spacious.
De-clutter
Large amounts of clutter on every surface of a room can make it seem far smaller than it really is, as an excessive number of items tends to create a crowded and cramped look.
Given this, one of the simplest and least expensive ways to create the illusion of spaciousness within your home is to de-clutter it. Spend a day or two going through your belongings and putting them into one of three piles: goods that can be donated, rubbish that needs to be either thrown away or recycled and things that can be sold.
As your main priority in this situation will be to make the rooms of your house look bigger, it's important to focus on removing visually accessible clutter (such as the items strewn across your kitchen table) first, rather than hidden clutter (like the unwanted clothes in your overstuffed wardrobe, for example). If you begin by de-cluttering your clothing collection or the clutter in the attic, you may get so absorbed in this process that you completely forget about your original goal.
As such, try to focus on removing old decorative knick-knacks, trinkets, old books, magazines and other 'stuff' that is cluttering up your tables, shelves and countertops; this should make a significant difference to how spacious and airy your home looks.
If you own items that are making certain areas of your home look very crowded and cramped, but you don't want to get rid of these goods because you will need them again at some point (things like Christmas decorations, gardening tools and fitness equipment might fall into this category), it might be worth putting them into a self-storage facility.
The storage units at these types of facilities are well-secured, easy to access (most self-storage businesses offer their customers 24-hour access to their belongings) and temperature controlled, and as such, they serve as a very safe place in which to store items that you don't want cluttering up your home all year round.
Maximise the amount of natural light in each room
A room which is filled with lots of natural light will look much bigger than one which is lacking in natural light, even if they are both the exact same size.
The two main ways to maximise the amount of natural light in your home is to paint the walls in light colours and to hang up a few large mirrors.
If the walls of a room are painted with neutral, pastel or plain white shades, they will reflect any natural light that enters in and, in doing so, will give it a bright, airy and spacious appearance. This painting process needn't be overly expensive, provided you are willing to do the painting yourself instead of hiring a professional decorator.
Like pale shades of paint, mirrors also help to reflect any light that enters through the windows of a room. Additionally, large mirrors can create visual depth, which creates the illusion that the room is significantly larger than it really is.