Estimate wifi router coverage by home size.
Wifi range figures on a box are measured in clear air, which bears little resemblance to a house. Each interior wall costs several metres of effective range and a floor costs more, because the signal crosses joists and often plumbing or underfloor heating. Subtracting those losses along the specific path you care about is far more useful than a single whole-home number, since coverage problems are always directional.
Range after obstruction losses
Total loss = walls x loss per wall + floors x 9 m; effective range = rated range - total loss; area = pi x effective range squared
About 3-4 metres for a stud and plasterboard partition, 6-8 metres for brick or block, and 15 metres or more for reinforced concrete. Foil-backed insulation and metal-mesh plaster are the worst offenders and can block a band almost entirely.
Yes, for the worse. Higher frequencies carry more data but are absorbed more strongly, so 5 GHz range through walls is roughly half that of 2.4 GHz and 6 GHz is shorter again. Enter the rated range for the band you care about.
Central, high and clear. Moving it out of a cupboard or off the floor often recovers more range than adding hardware, and keeping it away from microwaves, cordless phone bases and large metal appliances avoids the worst local interference.