Pinterest. Save everything you like then you can see the patterns of what you like.
Hardscaping/ anything structural and maintenance of boundaries are then a good next point. You don't want to have to replace a fence behind a newly established boarder or carry/ barrow heavy paving across a newly laid lawn.
So if your patio is to be extended that's a great starting point. Followed by making a decision on whether or not the lawn will be raised.
Will you have a rabbit open area/ section of the garden? If so think about how this closes off from the rest.
Next think about the bigger feature planting areas. It doesn't matter that budget doesn't stretch to big feature plants. Things grow so design in the space for them to do that and busy lives mean time passes really quickly. The 99p acer whip 5 years down the line is a lovely established feature. Likewise the bare rooted fruit tree give it five years and you'll have a small crop.
Right now is a great time for tucking in spring bulbs. Big bags of daffodils are around £5. If you pop them in, anywhere, they can be moved. So even if you just dig a bit of a trench and plant them, come spring they'll happily move in the green to the place you've had the winter to design. I live near a B and M stores and a homebargains. They often have bags of bashed bulbs/ ones which have started to shoot or lost their label for 10p. I just tuck them in some soil in a corner and see what happens - its low cost fun and I get a great buz from the something for almost nothing.
I buy lots of discounted plants like little roses that are rather sad in the supermarket, flowers died back so it's 30p. It's still a live plant deadhead and pop it outside in some soil before you know it it's a thriving small rose bush.
Gardens evolve. You don't need a final design. If you can work out a rough structure of the garden then just experiment with planting.
Cuttings are a great way to get plants for free. If you see a plant in a neighbours garden you like and can identify it then you can google how to propagate. If it doesn't work you've not really lost anything. Most people are quite flattered to be asked if you can take a cutting off their plant.
The most important thing is to enjoy the space.