Stamp duty: So you think you're a first-time buyer ...

First-time buyers (particularly those who don't drink cider) were among the biggest winners in the budget after Alistair Darling said they will not have to pay stamp duty on homes costing up to £250,000.

For many homebuyers, stamp duty is the biggest add-on cost, so this tax break – worth up to £2,500 – is not to be sniffed at. But who or what exactly is a first-time buyer?

Some might think that a daft question, but accountants say the taxman's definition is very strict, and that some buyers who might have assumed they were eligible may be surprised to discover that they are not.

The government says the £250,000 threshold, which took effect at midnight on Wednesday and will last two years, means nine out of 10 people buying their first home will not pay the tax. It was generally welcomed by commentators, who said it would put the dream of home ownership back within reach of thousands of hard-pressed homebuyers.

But many experts were quick to home in on the difficulties of defining a first-time buyer, and warned policing the system to ensure people do not abuse it was likely to be a headache for the authorities.

The Council of Mortgage Lenders, which represents the banks and building societies, cautiously welcomed the move but claimed that there will be "genuine practical barriers to effective implementation," and that it would clearly be far simpler – but more expensive – to exempt all properties under £250,000, rather than just those being bought by people who have never owned a home.

Here we answer the questions that readers might have.

So, what is HM Revenue & Customs's definition of a first-time buyer? It's not exactly snappy: "A person who has not acquired a freehold or leasehold interest in residential property in the UK (except a lease with less than 21 years to run) or an equivalent interest anywhere in the world." In other words, you don't qualify for the stamp duty exemption if you have owned a property in Britain or anywhere else in the world. HMRC adds the buyer(s) "must intend to occupy the property as their only or main residence". So it doesn't apply to second homes or buy-to-let properties.

I'm buying jointly with my partner. What's the deal? All the buyers, when there are more than one, must be first-timers, says Revenue & Customs. So you don't qualify for the stamp duty exemption if one of you has previously owned a home.

Many years ago, I bought a house jointly with my wife. We divorced a long time ago and I'm now with a new partner, who has never owned before. Can I/we be treated as a first-time buyer? As you have been involved in buying a home in the past, you won't be eligible for the stamp duty exemption if you decide to buy jointly. But if your new partner meets the criteria, she will be eligible if she buys the property in her name. Some couples won't want to go down that road for their own reasons.

What about someone previously in shared ownership: are they disqualified, even though they never bought outright? Yes, I'm afraid so.

I assume I'm excluded if my parents (who already own) buy a flat for me? (I've never owned a property) You're right, this transaction would not be eligible for the stamp duty exemption.

Can I claim the relief if I'm buying on behalf of my parents? No, says the Revenue. "Relief is not available unless the first-time buyer(s) are buying, for themselves, a property they intend to use as their only or main residence."

Can I get relief if I once inherited a property? After all, I didn't buy it. No, says Revenue & Customs. "In this case a person will be seen to previously have acquired a major interest in a residential property."

How will this regime be policed? What checks will be done? Anyone buying a property has to complete a stamp duty return form. You may not remember filling this is in at the time, but it was probably one of a stack of documents you had to sign. These forms are sent to Revenue & Customs, and that means you will be on its system as having bought a property. The Revenue says if you have previously bought a property, your name will flash up on the system.

What happens then? If you are found to have abused the system, in that you are not a genuine first-time buyer, you will be charged the stamp duty you have evaded, and be penalised with a fine of up to £5,000.

How will Revenue & Customs catch up with people who have owned a property overseas? The taxman boasts of having a vast network of "tax information exchange agreements" with other countries, which enable it to find out information about people who are new to the UK.

What if I change my name by deed poll? Surely I can't be found? "That's an interesting one," admits Revenue & Customs. However, if the taxman does catch you out, you will face the penalties outlined above.

Is there an age limit? No. First-time buyers can be any age to benefit.

I bought my home earlier this year and missed out on this exemption. Can I retrospectively make a claim? No. "Transactions with an effective date before 25 March 2010 do not qualify."

'The £1,600 saving will be really useful'

Tom Wicks, from Sidcup in Kent, is one of the many people in their 20s who won't benefit from the stamp duty holiday.

He's moving up from a two-bed flat to a three-bed semi he's buying with his girlfriend.

"I'm only 26, my girlfriend's a self-employed hairdresser and we don't exactly make a fortune. The new place is costing £205,000, so if we'd got the stamp duty back we'd have saved £2,050.

"What's infuriating is that we agreed to pay our buyers' stamp duty as delays meant they missed the last stamp duty holiday period.

"We're going to have to spend a lot doing the place up, so the cash would have come in useful."

'It's infuriating: we're paying our buyer's duty'

South Londoner David Cox, 27, said he will save about £1,600 from the temporary scrapping of stamp duty, as he struggles to get his foot on the property ladder.

He hopes to buy a part-share in the striking new Strata "Razor" building in Elephant and Castle, where one-bed flats are on sale via a housing association for £210,000-£250,000.

After paying rent for nine years, he plans to borrow £130,000 and, with additional help from his parents, buy a 60-70% share in the flat.

"The saving on stamp duty will be really useful. Although it's a small sum compared with the cost of the flat, it's cash in hand and will help with things like legal fees and furnishing."