After last years crisis of confidence the buy-to-let market is again booming. Earlier worries that interest rates were on the up and property values would crash are firmly behind us. So, fuelled by rising rental yields confidence, landlords have been snapping up new properties and remortgaging for cheaper deals.

In the final three months of last year, rental incomes increased by an average of 3.3%. At the same time the rental yield, income as a percentage of the property's value, edged up from 6.42% to 6.45%. The latest report from the Council of Mortgage Lenders (CML) also shows that the value of new buy-to-let mortgages increase by 47% in the second half of 2005 over the preceding six months whilst the number of these mortgages rose by 39%.

Indeed, we expect the boom to extend throughout 2006. It will be powered by the steady increases in house prices, a healthy demand from tenants, especially the first time buyers who remain priced out off the property ladder and a glut of cheaper buy to let deals.

Mortgage lenders are happy as well! Industry figures show that buy-to-let mortgages are now a safer bet for them than homeowner mortgages. According to the CML, percentage of arrears in buy-to-let mortgage is now lower than that for homeowner mortgages - and the arrears trend for buy-to-let is improving whist for homeowners it's getting worse.

Not surprisingly, the mortgage lenders have responded by relaxing some of their lending criteria and aggressively promoting buy-to-let again.

In the past, buy-to-let lenders have required monthly rental income to exceed mortgage payments by 30% – so if a mortgage was costing £750 per month, the rental income needed to exceed £975. But now several lenders have relaxed this criteria. The reason's not just the improved risk profile. Over the last six or seven years, house prices have risen faster than rental income yields, making it increasingly difficult for landlords to meet the +30% criteria. So now the lending average is closer to +25% although Northern Rock and a few others are happy to lend where the income simply equals the mortgage payment.

Simultaneously we have seen a trend for lenders to increase the percentage of the property's value they will lend on. Whilst 75% used to be the maximum level, the average is now closer to 85% with Northern Rock lending up to 87% and GMAC being prepared to stretch to 89%.

Interest rates on buy-to-let have also fallen. 4.75% is available from the Mortgage Trust on a three-year fix whilst 4.79% is available from the West Bromwich Building Society fixed for a two years. Both these deals incur a 1.5% arrangement fee. On the West Bromwich deal, when you recalculate the interest rate and include the arrangement fee amortised over two years, the equivalent rate rises to 5.54%.

Arrangement fees should not necessarily be a problem for landlords whose prime concern is cash flow. For these landlords it can be worth paying a large fee to obtain a low headline interest rate. That's because the rental income/mortgage payment calculation is based on the headline interest rate and this reduces the rental that has to be charged in order to meet the lenders income criteria.

If you're interested in joining the buy-to-let boom, remember to do your homework. Carefully research the local rental market - look at the rentals being achieved, the trends in property prices and levels of vacant to let properties.

And be especially careful especially if you're considering a city centre. Some lenders are becoming concerned at the potential oversupply of new flats and apartments in city centres they believe are becoming overpriced. Developers are responding by offering tempting cash back and discount schemes rather than reducing prices. But this can sometimes serves to mask the problem of over pricing. Realising this for some cities, lenders are reducing the value to lending ratio back to 75%.

Also remember that it's important to budget for the inevitable periods when the property is empty. In an essentially demand and supply market, if the rental market in your area becomes oversupplied you could be hit by lengthy vacancies or be forced to reduce your rental prices.