You generally can have hair extensions in any length of hair over three inches.
I have been extending hair for 12 years now and in my experience the shorter the hair the more extensions you need to make the blend between your own hair and the extended hair look natural, which in turn costs more. That being said, it all depends on how thick your own hair is and the current shape plus the desired effect.
The best brand available in my opinion is Great Lengths, as they are the market leaders and the hair quality is superb. You can visit their website www.greatlengthshair.co.uk to find a good salon near you and most will give a complimentary consultation.
I will warn you though not to go for a cheap option and do your research, as generally you only get what you pay for. I have seen some terrible results from the cheap options!
Good luck!
