In short: one describes the surface, the other what lies beneath
A topographic survey describes the surface of the plot: levels, slopes, actual boundaries, trees, existing structures, the street level and utility connection points. A geotechnical study describes the ground below: which layers it is made of, what load it can carry and where the groundwater is. Building a house needs both, and neither replaces the other.
From the topographic survey the architect sets the house on the plot and calculates earthworks. From the geotechnical study the engineer chooses the foundation type and designs the structure. Spain's technical building code (CTE) provides for a geotechnical study when designing foundations, so the question is not whether to do one, but when and how extensive.
What the topographic survey gives you
A plan with contours and spot levels on a grid, the true position of boundaries, which may not match the cadastre, tree trunks with their diameters, neighbouring structures near the boundary, manholes and poles in the street, and the pavement level at the future entrance.
Take a sloping plot in Castelldefels or the Garraf with several metres of fall from the street to the back boundary. The topographic survey is what tells you whether a garage entrance works without a steep ramp, how many retaining walls you will need and where the house sits with the least excavation. Without it all those decisions are made by eye and later corrected at great expense.
What the geotechnical study gives you
Boreholes and penetration tests at several points, a description of the layers with depth, bearing capacity, the water table if encountered, how aggressive the soil and water are to concrete, and recommendations on foundation type, founding depth, excavation slopes and whether dewatering is needed.
Near the sea it is often sand and high water; on slopes, rock next to made ground. Either way, the decision on foundations and how to excavate is made from this report. What that looks like on site is covered in the article on house foundations.
The order: surface first, then ground
The logical sequence runs like this. First the topographic survey. From it the architect produces the first siting options. Once the house position is broadly clear, the geotechnical study is commissioned, and the boreholes go where the building will stand, not in the most convenient corner of the plot.
If the house moves after the study, or a basement is added, ask the engineer whether the existing data is still enough. Sometimes one more borehole is needed. It costs less than foundations designed for ground that is not actually under the house.
How both studies affect the estimate
Two plots of the same size with the same house design can cost very differently up to ground-floor level. A change in level brings retaining walls and muck-away. Weak ground brings a raft or piles instead of pad footings. A high water table brings dewatering during works and serious waterproofing below ground.
That is why an estimate for building a house without these two documents is only indicative. Ask the contractor which data the foundations and earthworks were priced on, and record that in the contract.
Before buying the plot, or after?
A full geotechnical study is usually commissioned once the plot is yours and the siting is clear. But buying completely blind is not wise either. Before purchase, ask the seller whether they have a survey or any study, talk to neighbours who have built recently, and look for signs of old structures, filled-in pits or cracked retaining walls.
On a plot with difficult terrain or near water, it makes sense to commission the topographic survey before buying: it is cheap compared with the price of the land and shows straight away how much you will need to put into retaining walls and excavation. Access to the plot before completion is something to agree with the seller and your lawyer.
Common mistakes
Buying a plot without even a rough idea of what lies beneath it. Skimping on the number of boreholes. Borrowing the neighbour's report because it is next door. Not updating the data after the house moves. Treating the geotechnical report as instructions a crew can follow without an engineer.
The reports are interpreted by a qualified professional, and it is that person who should explain their effect on the design and the estimate. On house projects BCN YARD coordinates the exchange of information between the surveyor, geotechnical consultant, architect and engineer, so everyone works from the same version of the plot and the siting.