There are three values for any home on the market: What the seller thinks it’s worth, what the buyer thinks it’s worth and what a professional appraiser will think it’s worth. The key to a successful purchase is to get those three numbers to align.

When you’re trying to figure out how to value a house, there are some clues you can pick up right here on Zillow. We’re partial to starting with the Zestimate® home value, of course. Zillow’s Zestimate is based on data from sales of similar homes in the area. It’s the result of complicated and proprietary algorithms that are probably as accurate as you can get without actually walking into the house and looking around. But as great as it is, the Zestimate really is just a starting point. You’ll want to dig deeper before making an offer. The Zestimate doesn’t know about the brand new kitchen or the frayed, stained carpeting, both of which will impact the home’s value.

What are comps?

This is a real estate term you’ll hear from your agent and bank appraiser. A “comp” is a comparable home sale nearby. When you’re looking at a for-sale home on Zillow, you can see the price range of similar homes in the area that sold recently; just expand the “Price This Home” section. To get even more detail, your agent can help you compare houses of similar size with similar amenities that have sold within the past six months. A sale older than that won’t tell you much. In fact, some lenders will only look at sales within the past three months when it’s appraisal time.

This should be relatively easy in a hot market, with lots of sales or comps to choose from. In a slower market, or if the home has unusual amenities, you’ll have a harder time finding comps. Say you fall in love with a house on a lake, where houses almost never come on the market, making it tougher to find accurate comps. Or perhaps the house you are looking at has special design features that are rare in the area. Again, finding comps that make sense will be a challenge.

You also need to consider market activity. Perhaps lower-end homes are selling like crazy but large homes at the top end of the market are languishing, or vice versa. And don’t fall into the trap of thinking you can just look at “price per square foot” to determine a house’s value. By that measure, a condemned building would be worth as much as one in pristine condition, but the price per square foot varies by the size of the home as well. In other words, you’ll likely pay a little less per square foot for a 5,000-square-foot home than you would a 1,400-square-foot home.

The elements most critical to an accurate comparison are:

Location: If you have to look outside the immediate neighborhood to find comps, find a nearby area with homes of the same vintage and in other ways similar to the neighborhood where you are looking. Consider school districts. If looking at a house with a well-respected school system, don’t use comps from an area where the schools are not well-regarded.

Amenities: Does the home you’re looking at have a pool? A spectacular view? A guest house? If so, be sure to include comps that have those same amenities, even if you have to search a wider area.

Age: Houses built at the same time usually have similar amenities. Remodels will naturally alter that but some counties include an “adjusted year built” designation to account for major remodels or updates.

Sometimes you’ll find an unusually high or low comp that you can’t explain. It could be a sale resulting from a death or divorce that prompted a need to move quickly. Or perhaps there is some other circumstance skewing the price. You or your agent can contact the other agents involved to see if there is some explanation for a comp that is way off the mark.

What’s it worth to you?

Some things are just a matter of choice. If a swimming pool is factored into the price of a home but you plan to just fill it in and re-landscape, it doesn’t make sense to pay extra for it. If you love new carpeting, it’s worth it to pay a little more for a house with new, high-end wall-to-wall. But if you’re going to tear it out to install hardwoods, it’s not. If your idea of home cooking is popping something in the microwave, you probably don’t want to pay a premium for a gourmet kitchen when a nice, reasonably sized one would suit you just fine.

What’s it going to be worth to someone else?

Even if you plan on staying in your new home forever, circumstances change. You may want or need to sell it at some point, so you should also consider resale value. Some things to look out for are disclaimers about square footage not matching tax records. This is usually a signal that there has been an addition that was done without building permits. You may be fine with it, but that could impact resale value down the road.

What’s the appraiser got to do with it?

A lot. If you’ve fallen in love with a house and find yourself willing to pay a premium just to get it, you could still have a problem. Your mortgage lender will pay a professional appraiser to study comps and evaluate the home to come up with a value. If the appraiser comes in below the sales price you and the seller agree to, you have a problem. You’ll either need to cough up more cash upfront or renegotiate the price. Since the lender is using the home as security, they need to know it’s actually worth what you’re buying it for. That provides both another check and a motivation to both you and the seller to not go too far afield.