“Guesstimating” the value of a property

Any estate agent will happily value a house for you at no cost, in the hope of getting to sell it. It won’t be entirely accurate, because the only way to value a house accurately is to sell it and see how much you can get, but it will be better than your own guesstimate.

Depends on how much work you want to do.

The easiest way, is to take the price of the property at purchase, and multiply it by the ratio of the current average property price for [your town] and the average price at time of purchase (google can help you here). So if you bought it for 100k, and the average price has gone up by 10%, the current value is probably around 110k.

This won't be hugely accurate, because prices are fractal - a new shop built at the end of the street will affect prices on that street only, without shifting the average. So getting an accurate price will require research into recent events in your area, prices of nearby properties, changes in availability and quality of nearby properties... etc.

However, I'd worry more about the legal side. When you sell a property (or part of one) that you own, you are liable to capital gains tax, which is based on the difference between the selling price and buying price. This will be country specific, so you should consult a lawyer - you may find that increasing the price you sell for by X amount will put you over a cutoff and increase your tax liability by more than X.

Also worth checking, is whether your mother will have to pay tax when she gives it away. Again, I don't know about germany, but in the UK if you give away a property as a gift, you still have to pay the government capital gains tax on the difference between what you bought it for, and it's current value, even though you are not getting anything for giving it away.

You can use a comparative market analysis (CMA) by looking at the value of property in your immediate surrounding area.

Some factors you have to consider are:

Historic Sales

In different real estate markets, properties appreciate and depreciate at certain rates. You could start with looking at the past 12 months of sales to look at the historical sales value of properties in your area.

Economic conditions

This can have a significant effect on your home's value. If the unemployment rate is down and the economy is struggling, like we are going through now due to the coronavirus, the value of properties may decrease as a way to keep up with the decreased demand brought by these negative economic conditions.

Location

If you live in a good neighborhood, which has amenities and quick access to schools and other major infrastructure, this can affect the value of your property greatly. Another thing you have to consider is the style of your house and how it fares with certain demographics. a single family townhome will sell and be valued differently compared to a luxury condo.

These are just some of the many factors when it comes to rounding up an estimate of the value of your home. There are more factors concerning this strategic analysis, and you can read more about them here. In my opinion, however, appraising your property's value would be more accurate if you hire a professional and get consultations from an experienced individual in that industry. Good luck!