When the chancellor announced the changes to the Isa regime in March, it meant savers keen to use this year's £5,940 allowance early - putting in it a fixed-rate deal at the start of the tax year on 6 April - could miss out on the new higher allowance.
Banks and building societies don't normally allow you to add to a fixed-rate deal once it is opened. But they have now relaxed their rules to give Isa savers a window of opportunity to do just that. Check what your provider offers to ensure you don't miss out.
Sue Hannums, director at savings advice site Savingschampion.co.uk, says: "Savers may be bewildered by the differing deadlines. Some plans let you top up at the original rate, while others ask you to open a new fixed-rate deal."
Some, including Leeds, Yorkshire and Skipton building societies, only give you until the end of July to act. And if you opened a fixed-rate deal this tax year with NatWest you could have only until 18 July to top it up.
Coventry BS will let you top up its 2.75% deal, fixed to May 2018, but it will not disclose how or when until sometime in June. Lloyds works differently again. With its 2% fixed for two years, you can top up until the end of the tax year.
Find the best cash Isa or savings account for you
Under Halifax new rules, you have 180 days to top up its fixed-rate deals opened in April, May or June. These include a 2% deal for 18 months or 2.05% for two years. With Santander's two year deal at up to 2.3%, you only have until 31st August to act.
At Nationwide, where you can earn 2.05% for two years, you can top up during July - or open a new account any time during the tax year.
This article was written for our sister website Money Observer