Yes, and with fewer obstacles than in most places that call themselves a village. There are two listed buildings. There's no conservation area. The ancient protected sites are out in the fields, away from the houses. Your biggest planning obstacle is likely to be the neighbour whose kitchen window faces yours.
Historic England lists two buildings in Mortimer, and both are Grade II. There are no Grade I or Grade II* buildings. Listing means that almost any change to the building, inside or out, needs listed building consent on top of ordinary planning permission, and replacing old windows or knocking through a wall turns into a case for the council. With only two such buildings, the chance that the house you're viewing is one of them is small, and the listing will be in the sale paperwork if it is. For almost every other buyer, the rule doesn't apply.
The surrounding area also holds four scheduled monuments, the strictest tier of protection, under which even digging needs national consent. They matter to the few properties with land running up to one. For a typical back garden, they won't come up.
The planning register shows people are making full use of that freedom. The area logged 1,437 planning applications between April 2024 and June 2026, including 632 in the last twelve months. That's a steady flow of loft conversions, rear extensions and garage rethinks, and it is the most honest record of what locals think their houses are missing. Somebody on your road has almost certainly applied for something already, and their application sits on the council's register for anyone to read before you draw up your own.
In practice, the lack of a conservation area means that many single-storey rear extensions, loft conversions and outbuildings can go ahead under permitted development rights, the national allowance for modest changes without a full planning application, within set size limits. Anything larger, anything at the front, or anything the rules don't cover needs full planning permission from West Berkshire Council. Newer estates sometimes have those rights removed by a condition on the original planning consent, so check the property's own planning history before assuming. Building regulations apply either way, and building control will want to see what was built.
For anyone who wants to extend, Mortimer is one of the easier places to buy. Heritage rules barely apply, and the main constraints are the ordinary ones that come with any house.